Organic Photocatalysts for the Oxidation of Pollutants and Model

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Organic Photocatalysts for the Oxidation of Pollutants and Model Compounds M. Luisa Marin,† Lucas Santos-Juanes,‡ Antonio Arques,‡ Ana M. Amat,‡ and Miguel A. Miranda*,† †

Instituto Universitario Mixto de Tecnología Química-Departamento de Química (UPV-CSIC), Avda. de los Naranjos s/n, E-46022, Valencia, Spain ‡ Departamento de Ingeniería Textil y Papelera, Universidad Politecnica de Valencia, Campus de Alcoy, Plaza Ferrandiz y Carbonell s/n, E-03801 Alcoy, Spain

CONTENTS 1. 2. 3. 4.

Introduction Organic Photocatalysts: Structures and Properties Pollutants and Model Compounds Reaction Mechanisms 4.1. Experimental Mechanistic Evidence 4.2. Photodegradation and Identification of Photoproducts 5. Photocatalytic Applications 5.1. Organic Photocatalysis in Heterogeneous Media 5.1.1. Preparation and Characterization of Supported Organic Photocatalysts 5.1.2. Photocatalysts Stability and Reuse 5.1.3. Efficiency 5.1.4. Organic Versus Inorganic Photocatalysts 5.2. Environmental Applicability: Use of Solar Light, Scale-up, Detoxification, and Biodegradability 5.3. Tabular Survey of the Endpoints Addressed with Different Photocatalyst/Pollutant Combinations 6. Summary and Outlook Author Information Biographies Acknowledgment References

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1. INTRODUCTION Water constitutes a key resource for life on Earth; as a consequence, development of civilizations throughout history has been directly related to the availability of sufficient sources of water with the required quality.1 Despite the abundance of this resource, its actual availability is limited by the following facts: (a) ca. 97% is found as seawater and 2/3 of the rest is immobilized in poles and glaciers, (b) neither is water homogeneously disseminated nor are all the lands equally populated, and c) human activities have resulted in the generation of increasing amounts of wastewaters, which do not meet the required standards of quality to be reused and may cause pollution of the aqueous systems where they are discharged. Therefore, water scarcity constitutes an increasing concern, whose solution is a major challenge.2 r 2011 American Chemical Society

An opportunity to increase water availability is decontamination and reuse of wastewaters. Much work has been recently devoted to this issue, and in developed areas, most household and urban effluents are processed in conventional sewage treatment plants. They generally consist of a physicochemical pretreatment followed by a biological step and, eventually, a tertiary treatment to ensure disinfection and decontamination.3 However, further scientific and technical work is still required because a number of chemicals are not amenable to bioremediation, owing to their reluctance to conventional treatments or even to their toxicity to the biological systems.4 Chemical oxidation constitutes an alternative to bioprocesses for degradation of toxic or nonbiodegradable species. 57 Ozonation,8,9 wet oxidation,10 hydrogen peroxide-based methods,11 and electrochemical12,13 or photochemical oxidation have been used for this purpose. In particular, the applicability of photochemical processes for decontamination of water and air has received considerable attention over the last few decades. Some reviews and monographs have been devoted to report general advances in this field,14,15 and others have dealt with the use of this type of methods to remove specific pollutants such as methyl tert-butyl ether,16 chlorophenols,17 dyes,18 or pesticides.19 A variety of radiation sources with different wavelength ranges have been used: (a) VUV-radiation, with λ < 200 nm, to decompose pollutants by direct photolysis or to generate oxidizing species such as hydroxyl radical ( 3 OH) or ozone from water or oxygen,20 (b) UVC (commonly with λ = 254 nm, produced by low pressure mercury lamps), which is also able to photolyze a number of compounds, thus enhancing the effect of ozone or hydrogen peroxide by formation of 3 OH,14,21 and (c) UVBUVAvisible, which usually requires the presence of a photocatalyst and corresponds to the fraction of the solar spectrum reaching the Earth’s surface. In this context, photocatalysis is an interesting approach for photochemical detoxification. According to the IUPAC,22 a photocatalyst is a substance that is able to produce, by absorption of light quanta, chemical transformations of the reaction participants, repeatedly coming with them into the intermediate chemical interactions and regenerating its chemical composition after each cycle of such interactions. Thus, the photocatalyst must be efficient in substoichiometric amounts. Accordingly, photocatalysis is defined as a “change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substance—the photocatalyst—that absorbs light and is involved in the chemical transformation of the reaction partners”. The Received: February 15, 2011 Published: October 31, 2011 1710

dx.doi.org/10.1021/cr2000543 | Chem. Rev. 2012, 112, 1710–1750

Chemical Reviews term photocatalyst is closely related to photosensitizer, which is defined as an agent that absorbs light and subsequently initiates a photochemical or photophysical alteration in the system, with the agent not being consumed therewith. In the case of chemical alteration, the photosensitizer is usually identical to a photocatalyst. Photocatalysis can be considered as one of the emerging green processes, which has been employed for different purposes such as hydrogen production, organic synthesis, and water or air decontamination.2325 In particular, photocatalysts employed in pollution remediation are able to generate highly oxidizing species upon excitation, which are able to react with the contaminant molecules. Most of these processes can benefit from the additional economical and ecological advantages associated with the use of freely available, natural sunlight.26 This topic

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is gaining momentum, as indicated by a rather conservative estimation of ca. 1500 papers on photocatalysis per year, 50% of them evaluating the possibility of using sunlight. Specific information on the use of sunlight for wastewater detoxification can be found in a series of review articles summarizing recent advances in the field.2630 Photocatalysts employed for air and water decontamination can be divided into two groups: (a) semiconductors, such as metal oxides and sulfides, and (b) organic compounds and metal organic complexes with strong absorption bands in the UVA visible range, capable of participating in photocatalytic processes. Semiconductors probably constitute the most widely used heterogeneous photocatalysts. Much work has been devoted to elucidate their mechanism of action; detailed information on this

Chart 1. Structures of Photocatalysts

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Chart 1. Continued

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Chemical Reviews issue is available elsewhere.3133 Briefly, electrons are promoted from the valence band to the conduction band upon irradiation, thus generating a high-energy electron and a corresponding oxidizing hole (h+) in the valence band; these species can react directly with the pollutant, or give rise to secondary oxidizing species, such as hydroxyl radical. Although different materials have been tested, in most cases titanium dioxide is the photocatalyst of choice. The role of this solid was established in 1972 by Fujishima and Honda,34 and the principle was first employed for decontamination by Carey et al. in 1976.35 Since then, a wide range of pollutants and wastes have been successfully treated by TiO2-driven photoprocesses, such as dyes,18,36 volatile organic compounds,37 chlorophenols,17 or pesticides;38 furthermore, a specific review has appeared on the fundamentals of TiO2 solar photocatalysis and solar photoreactors.39 Current efforts in the field are being devoted to optimize the efficiency of these processes,40 mainly by enhancing sunlight absorption (only λ < 380400 nm can be used, which accounts for P1k > P1o.176 Fluorescence quenching by bisphenol derivatives 1715


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Chart 2. Structures of Pollutants and Model Compounds

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Chart 2. Continued

P2ac has been assessed by steady-state and time-resolved techniques. A positive curvature in the SternVolmer plot obtained in the steady-state experiments indicates a dark association between the bisphenols and ground-state riboflavin; by contrast, a linear plot is obtained for the time-resolved data, affording dynamic quenching constants of 5.32, 2.93, and 5.99 109 M1 s1.141 As regards heterogeneous photoreactions, oxidation of parathion-ethyl (PTE) photosensitized by an anthracene (ANT)-substituted dextran involves a photoinduced electron transfer process. Energy transfer from the ANT singlet excited state (3.31 eV) to PTE (4.53 eV) is unfavorable; nevertheless, the photosensitizer excited state is quenched efficiently (kq = 4.1 109 M1 s1).102 Likewise, fluorescence of perylene bisimide PBIa is quenched by increasing concentrations of cinnamic acid (A5a) with a rate constant above the diffusion limit (8.7 1011 M1 s1).83 Participation of 1P* in an electron transfer mechanism is also supported by the static quenching of TPP fluorescence by benzoic acids A3ch and by the parallel shortening of its singlet state lifetime. The corresponding SternVolmer plots have been obtained (Figure 2); the rate constants calculated therefrom are in the range 1.5 1010 to 6.6 1010 M1 s1.186

The good correlation obtained within a family of pollutants, such as cinnamic acids A5ad, between the kinetic values (3.6 1091.0 1011 M1 s1) and the calculated thermodynamic parameters constitutes additional evidence for the involvement of an electron transfer process (Figure 3).191 Photoinduced electron transfer involving the singlet excited state of phthalocyanines PC1g and PC1l has been postulated in the photodegradation of phenols P1a, P1b, and P1g.148 An absorption maximum at 590 nm appearing immediately after the laser pulse indicates formation of the PC anions. Furthermore, the triplet state of phthalocyanines PC1g and PC1l (λmax = 490 nm) is not significantly affected by the presence of these phenol derivatives. Further insight into mechanistic pathway i has been obtained by laser flash photolysis (LFP) detection of the generated species. Thus, reaction of the singlet excited state of N-methylquinolinium (1NMQ*) with sulfides S1c and S1h occurs with rate constants of 1.8 and 2.4 1010 M1 s1, respectively.104 Analysis of the signals obtained by LFP in the presence of S1h reveals formation of three transient species (Figure 4). The absorption band with maximum at 520 nm is attributed to the pollutant radical cation (PhSMe) 3 +; the 1717


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Table 3. Photocatalysts Used for the Oxidation of Specific Pollutants photocatalysts pollutants

pyryliums

aromatics

heteroaromatics

chlorins and porphyrins

‑

PC1a152 PC1d146 PC1e147 PC1f147 PC1h147 PC1l147,155 PC1m146,155 PC1n146,155 PC1o155 PC1p155 PC3a155 PC3b155 PC1d146 PC1h95 PC1l95,155 PC1m95,146,155 PC1n146,155 PC1o155 PC1p155 PC295 PC3a155 PC3b155

CN Na2S

Na2S2O3

NDMAa

PP2b90 PP2n90 PP2b90 PP2n90

DCANab S1a

S1b S1c

S1d S1e S1f S1g S1h S1i S1j S1k S1l S1m S1n

TPP156,157 TPTP157 TPP109

ANT106 DCA63,106,107 AQ63,106 DCA156 ANT109 DCA104,105,107,109 DCAC107 AQ109

TPP107,156,157,159 TPTP157,159 TPP160

DCA107,156 DCAC107

TPP157 TPTP157 TPP161 TPP157,161 TPTP157 TPP161 TPP161 TPP161

DCA104

NMQ156 NMQ104 RB109

MB158 MB158 NMQ156 MB158

NMQ104

DCA161 DCA161 DCA161 PC1g153 PC1k153 PC1h95 PC1l95 PC1m95 PC1n95 PC295 DCA63 AQ63 ANT109 DCA105

S2a S2b

S3 S4

PC1b160 PC1c160 PC1g153 PC1k153

DCA161 DCA161

S1o

S2c

phthalocyanines

TPP159 TPTP159 TPP116

NMQ116 MB116

TPP159 TPTP159 1718


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Table 3. Continued photocatalysts pollutants S5 S6 S7 MTDT PTM PTE P1a

P1b

pyryliums

aromatics

TPP159 TPTP159 TPP159 TPTP159 TPP116

heteroaromatics

TPP162 TPTP162 TPP163,164 ANT102 TPP68,165,166 TPTP165,166 BP68 TPP173

DCA173

PBIc167 RB167169 RF123,170 MB167 RB173 RB124 MB124 RB168 RF123 RB80,143,167,168,175 RF176 MB143 RB167,168 RB167,168 RF123

P1d P1e

P1f P1g

P1h P1i P1j

TPP173,185

P1k

TPP218

DCA173 ANT102

P1o P1p P2a P2b

P2c P3a

P3b

PP2e167 PP2f172

PP2k174

PC1f167,172 PC1g148,171,172 PC1j167 PC1l148,167 PC1g148 PC1l148

CH2143

PC1f167

PP2b90 PP2j88 PP2k150 PP2l88 PP2m88 PP2n90

PC1f167 PC1f167 PC1g94,148,171 PC1k177 PC1l94,148 PC1o94 PC1p94

RB173 RF123 RB168 RF123

PC1g171 PC1l154 PC1p154 PC1g171 PC1k177

RB168 RF170,176 PP2j88 PP2l88 PP2m88

P1l

P1n

phthalocyanines

NMQ116 MB116 AYG162

P1c

P1m


RB124 MB124 RB178 MB178 RB168 RF170,176 RB168 RF170 RB141 RF141 RB141,181 RF141 MB181 RB141 RF141 RB124,182 RF170 MB124,182 RB182 MB182

PC1g179 PC1g171 PC1k98,177,180 PC1g179 PC3a97 PC3b97

CH1130 PP2a130 PP2c130 PP2d130 PP2f183 PP2i130

P3c

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Table 3. Continued photocatalysts pollutants

pyryliums

aromatics

heteroaromatics

TPP185 TPP186

A3c

TPP173,186

DCA173

A3d A3e

TPP186 TPP173,186

DCA173

A3f A3g

TPP186 TPP186

A3h A4a A4b

TPP186 TPP188

A5a

TPP189191

A5b

TPP173,189191,193

DCA173

A5c

TPP173,189192 TPTP192 TPP67,189192,194,195 TPTP67,192

DCA173

C1a C1b C1c C2 CBR

MB186 PC1g153 PC1k153

RA67

TPP

TPP TPTP67,162

DMA199

H3b

DMA199

H4

H7e

PC1q67

RF197 AYG162

67,162

H2c H3a

H7d

PP2k174

AQ110

DMA199 DMA199

H7c

PP2k174

AQ110

H1b H1c H1d H1e H1f H1g H1h H2a H2b

H7b

PBIa83 MB191 RB173 MB191,193 RB173 MB191 AYG67,74 MB67,191

PP2b90 PP2n90

196

DMA199

H6 H7a

RB173 MB186 MB186 RB173 MB186 MB186 RF187 MB186 MB186 PBIb72

H1a

H5a H5b H5c

phthalocyanines

MB184

A1 A2 A3a A3b

A5d

chlorins and porphyrins PP2b90 PP2n90

P3d

DMA199

RB144,198200 RF144,200 RB144,198 RB144,198 RB144,198 RB144,198 RB144,198 RB144,198 RB144 RB144,199 RB144,199,200 RF144,200 RB144 RB144,199 RF144 RB144,199 RF144 RB142 RF142 RB144,199,201 RB144,201 RB144,201 RF144,202 PBIb72 RB121 RF121 RB121 RF121 RB121 RF121 RB121 RF121 RB121 RF121 1720


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Table 3. Continued photocatalysts pollutants

pyryliums

aromatics

ATZ

ATT AMT

RF204 RF204

TDM M1 M2

TPP207

DCA207 AQ110

DDT M3a M3b M3c M3d M3e M3f M4 M5 M6 M7

TPP165,166 TPTP165,166


PP1205 PP2f206 PP2j206

phthalocyanines

PC1i205

PP2f206 PP2j206

RB207 RB208 RF208 MB208 RF209

TNT

PP2f210 PP2g210 PP2h210

MG211 RF170 RF170 RF170 RF170 RF170

TPP212,213 PP3151 RF214,215 TPP216

M8 M9a M9b M10a M10b a

heteroaromatics RB121 RF121 RF203,204

H7f

RB217 RF217 RB129 RB129 RB129 RB219 RF219 RB219 RF219

Nitrosodimethylamine. b Sodium dichloroacetate.

reduced form of the photocatalyst NMQ 3 , with maximum at 550 nm, and the dimeric form of the radical cation (PhSMe)2 3 +, peaking at 780 nm, are also involved (Scheme 2).104 Similar results are obtained from the LFP of NMQ in the presence of sulfide S1b that results in a broad band with maximum at 525 nm, due to the overlap of the radical NMQ 3 absorption (λmax = 550 nm) and the sulfide radical cation dimer (S1b)2 3 + (λmax = 485 nm).156 In the case of Ph2S (S1f), the transient absorption spectrum contains the NMQ 3 band (550 nm) and the S1f 3 + trace centered at 740 nm. Photooxygenation of sulfides such as S3 and S7 is mechanistically complex and strongly dependent on the photocatalyst used. In fact, in the presence of NMQ or TPP, the reactions display the characteristics of electron transfer photooxygenations. Thus, both photocatalysts are quenched by S3 and S7 at a diffusion rate constant, and the LFP experiments reveal the presence of NMQ 3 or TPP 3 , together with the sulfide radical cations.116 In addition, RehmWeller calculations indicate that electron transfer is indeed exergonic. However, the diverging photoproduct patterns suggest the possibility of different photooxygenation pathways. Hence, a new mechanism has been proposed involving addition of oxygen to TPP 3 with formation of two peroxy radicals. These intermediates react with sulfide radical cations, forming two persulfoxides that subsequently decompose affording thiadioxirane.116

Only a few reports have appeared on the involvement of OH 3 (pathways ii and iii) in the oxidation with organic photocatalysts;163,164,188,221 one of them deals with the photodegradation of 4-chlorophenoxyacetic acid (A4a)188 or methylparathion (PTM)163,164 by TPP encapsulated in zeolite Y. Photodegradation of PTM leads to paraoxon and 4-nitrophenol. An electron transfer mechanism from the singlet excited state of TPP (Eox = 2.5 V vs SCE)43 can be safely ruled out due to the high Eox of PTM (2.6 V vs SCE). However, the involvement of hydroxyl radical has been inferred from the following experiments: (a) generation of OH 3 in the photolysis of TPP encapsulated within zeolite Y in the presence of methyl viologen gives rise to an absorption band centered at 470 nm that corresponds to the OH 3 adduct of methyl viologen; (b) irradiation of heterogeneous TPP in the presence of 5,5-dimethyl-1-pyrroline-N-oxide generates an EPR signal at g = 1.997, attributable to the formed OH 3 adduct; (c) formation of hydrogen peroxide and suppression of the photoreaction in CDCl3 provides a further piece of evidence for the involvement of hydroxyl radical. Moreover, irradiation of a model compound, benzhydrol, in the presence of TPP in zeolite Y gives only benzophenone. In fact, the reactivity of OH 3 radical with different pollutants has been quantitatively determined by competition experiments, looking at the decrease of the typical transient absorption of the stilbene adduct at 390 nm. Figure 5 shows the formation and 1721


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Scheme 1. Alternative Mechanistic Pathways

Figure 3. Semilogarithmic plot of kq versus ΔG associated with photoinduced electron transfer from phenolic pollutants (A5a, A5b, A5c, and A5d) to the singlet excited state of TPP. Adapted with permission from ref 191. Copyright 2000 Elsevier BV.

Figure 2. (Top) Quenching of the TPP fluorescence by the phenolic pollutant A3h at several concentrations (up to 2.5 103 M) in acidic aqueous medium. (Bottom) SternVolmer relationship between the emission intensity and the concentration of A3h. Adapted with permission from ref 186. Copyright 2001 Elsevier BV.

decay of this band in the presence of increasing concentrations of methidation (MTDT). Comparison of the SternVolmer slope (Figure 5, bottom) with that of a standard (naphthalene) yields a rate constant of 7.1 109 M1 s1 for MTDT in acetonitrile, close to the diffusion limit.221 Electron transfer from photocatalyst triplet states to pollutants, pathway iv, also occurs.67,88,121,141,142,150,192,200,202,217 For

instance, triplet riboflavin, 3RF*, is quenched by benzimidazoles H7a, H7b, H7d, H7f, and 17β-estradiol (M7) efficiently; the lifetime of the broad absorption at 600700 nm (3RF*) is shortened in the presence of pollutants, with concomitant production of a new band at 530 nm, due to RF 3 .121,217 Even more, the transient spectrum of RF in the absence of pollutants shows a broad absorption with maximum absorbance at 600700 nm assigned to 3RF*; in the presence of pollutants, the 3RF* lifetime is shortened with concomitant production of a new band centered at 530 nm, due to RF 3 ‑. Disappearance of the 3RF* signal has been monitored at 670 nm in the presence of increasing concentrations of pyridine H2b, norflurazon (H4), and pyrimidine H5c.142,200,202 From the SternVolmer plots the calculated quenching rate constants are 1.2, 5.9, and 2.7 107 M1 s1, respectively. Again the shape of the transient spec trum with maximum at 530 nm recorded in the presence of H2b, due to RF 3 , further confirms the electron transfer process (Figure 6). The interaction of 3RF* with bisphenols P2ac has also been studied by LFP; the bimolecular rate constants range between 1.4 and 2.1 109 M1 s1. In the presence of P2ac, the signal ascribed to the triplet excited state is replaced by a long-lived absorption due to the semiquinone radical RFH 3 arising from protonation of RF 3 .141 In other examples, TPP and triphenylthiapyrylium (TPTP) are very efficient photocatalysts for degradation of cafeic (A5c) and ferulic acids (A5d).67,192 In fact, both photocatalysts are quenched by A5cd in CH3CN with rate constants near diffusion control (kq ca. 1 1010 M1 s1). Moreover, the fluorescence of 1TPTP* is efficiently quenched, in water, with a constant of 2.8 108 M1 s1. However, since the intersystem crossing quantum yield is ca. 0.94 for TPTP, involvement of the excited triplet state seems more likely. Actually, LFP experiments show a transient with broad maximum between 450 and 700 nm, attributed to the T1Tn absorption of TPP (Figure 7, top) or TPTP (Figure 8, top). Upon addition of increasing concentrations of A5c and A5d, a faster decay of the triplet is observed than in the case of TPP, concomitant with the appearance of a new band with maximum at 550 nm, assigned to the pyranyl radical (Figure 7, bottom). Likewise, the triplet lifetime of TPTP progressively decreases upon addition of increasing concentrations of A5c or A5d (Figure 8, bottom). Quenching constants determined for deactivation of 1722


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Scheme 2. Postulated Mechanism for the Photooxidation of S1h by NMQ (Adapted with Permission from Ref 104; Copyright 2003 American Chemical Society)

Figure 4. Time-resolved absorption spectra obtained upon LFP (λexc = 355 nm) of NMQ (3.5 103 M) and the sulfide S1h (1 103 M) in deaerated CH3CN. Adapted with permission from ref 104. Copyright 2003 American Chemical Society.

triplet excited TPP and TPTP by A5c and A5d are shown in Table 4. Thermodynamic calculations using the RehmWeller equation indicate that in the four cases photoinduced electron transfer is feasible from both excited states. Nevertheless, under the reaction conditions employed, quenching of the singlet state is 50% in the case of TPP and up to 90% for TPTP. Taking into account the appearance of the pyranyl radical for TPP, concomitantly with triplet decay, photodegradation of A5cd is mainly mediated by the triplet state of the photocatalysts. Further examples of photocatalysts acting through an electron transfer mechanism from the triplet excited state (pathway iv, Scheme 1) are provided by porphyrins. For instance, upon excitation of porphyrin PP2k at 355 nm, a transient absorption with maximum at 460 nm (see Figure 9) has been ascribed to the triplet state. Its decay has been monitored in the presence of increasing concentrations of 4-chlorophenol (P1g); from the SternVolmer plot, a quenching rate constant of 1.68 106 M1 s1 has been determined.150 Additional experiments indicate that triplet quenching of different porphyrins by P1g is strongly dependent on the nature of the central metal. Thus, for the free base porphyrin (PP2j) kq < 106 M1 s1, whereas for PP2l and PP2m the values are 2.2 and 1.9 107 M1 s1, respectively.88 Participation of 1 O 2 , pathway v, 94,97,106,116,121,124,129, 130,144,147,150,153,154,158,161,168,172,178,181184,198,199,201,217,222,223

in the methylene blue (MB) photocatalyzed degradation of sulfide S1d,158 phenols P1c124 and P1n,178 naphthol P3a,124 or benzaldehyde A1,184 has been inferred from experiments performed in the presence of typical singlet oxygen quenchers (NaN3 or DABCO). Likewise, retarded photodegradation of 1,5-dihydroxynaphthalene (P3c) by PP2f in the presence of NaN3 suggests the involvement of 1O2 in the process.183 In the photooxidation of S1a or S1im sensitized by 9, 10-dicyanoanthracene (DCA) or anthraquinone (AQ), participation of singlet oxygen has been postulated based on comparison of the photoproducts obtained with different photosensitizers.106,161 Several phthalocyanines have been employed in the photodegradation of sulfides and phenol derivatives. The reaction mechanism depends on the nature of the coordinated metal ion. In the case of phthalocyanines PC1e, PC1f, and PC1l, participation of 1O2 in the oxidation of Na2S has

Figure 5. (Top) Kinetic traces due to stilbene-OH 3 upon addition of increasing concentrations of MTDT (1 103 to 3.5 102 M) in deaerated acetonitrile. (Bottom) SternVolmer plots for MTDT (() and naphthalene (O) (standard) versus pollutant concentration. Adapted with permission from ref 221. Copyright 2011 Elsevier BV.

been assumed from the experiments performed in the presence of sodium azide, whereas for PC1h an electron transfer mechanism where Co(II) is reduced to Co(I) seems to be supported by the appearance of additional absorption bands.147 Phthalocyanines PC1g and PC1k have also been employed for the photooxidation of methyl phenyl sulfide (S1h), 2-propenyl sulfide (S1n), and 2-mercaptobenzoic acid (A3b), which is initiated by singlet oxygen as revealed by the effect produced by sodium azide (singlet oxygen quencher) and benzoquinone (superoxide scavenger).153 Photodegradation of P1g in the presence of different photocatalysts, such as PC1g, PC1l, PC1o, and PC1p, proceeds through a 1O2 mechanism; however, direct interaction between the singlet excited 1723


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Figure 6. Transient absorption spectra of RF (1 105 M) in argonsaturated aqueous solution in the absence (black) and in the presence (red) of H2b (8 103 M). Adapted with permission from ref 200. Copyright 2001 Elsevier BV.

Figure 8. (Top) Transient absorption spectra obtained upon LFP (λexc = 355 nm) of TPTP (7 105 M) in deaerated CH3CN at different times after the laser pulse. (Bottom) Decay of 3TPTP* obtained upon LFP at 355 nm, monitored at 490 nm, upon addition of increasing concentrations of A5d (01.7 104 M). Adapted with permission from ref 67. Copyright 2007 Elsevier BV.

Table 4. TPP/TPTP Triplet-State Quenching Constants by the Model Pollutants A5c and A5d

Figure 7. (Top) Transient absorption spectra obtained upon LFP (λexc = 355 nm) of TPP (7 105 M) in deaerated CH3CN, in the absence (9) and in the presence (b) of the phenolic pollutant A5d (4.7 104 M). (Bottom) Decay and growth traces monitored at 450 (3TPP*) and 550 (TPP 3 ) nm, respectively, in the presence of A5c (3.5 105 M). Adapted from ref 192 by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC.

state of the photosensitizers and P1g is also possible. Experiments at different concentrations and in the presence of NaN3 show that the relative contribution of both mechanisms is concentration dependent.94 When a mixture of sulfonated Zn phthalocyanines is used, the photooxidation of P1g follows mainly a 1O2 mechanism, as indicated by the use of 9,10-dimethylanthracene (DMA) as a

kq (M‑1 s‑1)

3

photocatalyst

model pollutant

TPP

A5c

1.3 1010

TPTP

A5c

4.9 109

TPP

A5d

9.7 108

TPTP

A5d

8.9 109

quencher.222 Involvement of singlet oxygen in the photodegradation of other phenol derivatives, such as P1j, in the presence of PC1l, PC1p, or a mixture of sulfonated Zn phthalocyanines, has also been confirmed using NaN3.154 In the heterogeneus photodegradation of phenol (P1a) using porphyrin PP2f or phthalocyanines PC1fg, participation of 1O2 (as well as O2 3 ) has been proposed based on the effects observed in the presence of additives.172 Stronger evidence for the generation of 1O2 has been provided by different spectroscopic techniques. For instance, in the case of PC3b,97 electron paramagnetic resonance (EPR) evidence has been obtained using 2,2,6,6-tetramethylpiperidine (TEMP) as singlet oxygen probe. Thus, Figure 10 shows that visible light 1724


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Figure 10. EPR spectra obtained after irradiation of a solution of TEMP (102 M) in the presence of PC3b (red trace). In the absence of PC3b or in the dark (control black trace), only the baseline is recorded. Adapted with permission from ref 97. Copyright 2005 Elsevier BV.

Figure 9. (Top) Transient absorption spectra of PP2k (6.2 104 M) obtained after laser flash excitation at 355 nm in deaerated water at different times after the laser pulse. (Bottom) Kinetic plot of the reciprocal triplet lifetime versus chlorophenol P1g concentration. Adapted from ref 150 by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC.

irradiation of PC3b in aerated medium gives rise to a 1:1:1 triplet signal, characteristic for the reaction of 1O2 with TEMP.97 In a control experiment, no signal is observed in the absence of photocatalyst or in the dark. For direct detection of 1O2, time-resolved near-infrared emission at 1270 nm is necessary. Thus, when rose bengal (RB) is excited at 532 nm, the phosphorescence emission of 1O2 at 1270 nm can be recorded in the absence and in the presence of pyridine H1b, in neutral and alkaline media (Figure 11).198 The SternVolmer plots provide a higher quenching rate constant in alkaline medium, as the conjugate base is more easily oxidizable. Other pollutants, like phenols P1e, P1f, P1g, P1j, P1k, P1o, and P1p; 17β-estradiol (M7); and neonicotinoids (M8 and M9ab) can be oxidized by 1O2, generated by irradiation of RB.129,168,217 Correlation between the determined quenching constants and the oxidation potentials reveals a quantitative structureactivity relationship.168 A number of heteroaromatic pollutants also react with 1O2. Thus, rate constants for quenching of singlet oxygen by H1ah, H2ac, H3ab, and H5ac have been determined by timeresolved phosphorescence.144,199,201 In the case of H7af, the value obtained is ca. 106 M1 s1.121 In a further example,

involvement of 1O2 in the photooxidation of bisphenol P2b using either RB or MB as photocatalysts has been proven by phosphorescence quenching with kq in the range 104105 M1 s1, depending on the aggregation. The radicals resulting from reaction of P2b with 1O2 are detected by EPR.181 Photooxygenation of sulfides such as S3 and S7 in the presence of MB follows a typical singlet oxygen pathway, which is supported by the following pieces of evidence: (i) quenching of the 1270 nm emission of 1O2; (ii) suppression of product formation in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO); (iii) isotope effect consistent with a persulfoxide intermediate; and (iv) lack of reactivity of the sulfur directly attached to the two phenyl rings, in accordance with the behavior of Ph2S under singlet oxygenation conditions.116 Porphyrin photocatalysts are known to generate 1O2. This species, obtained by irradiation of PP2k (ΦΔ ≈ 1), reacts with p-chlorophenol (P1g) with kq = 6.0 106 M1 s1. Therefore, although this phenolic pollutant is able to quench the porphyrin triplet state with 3kq = 1.7 106 M1 s1, its main photodegradation pathway involves reaction with singlet oxygen.150 By following the 1270 nm emission decay at different 1,5dihydroxynaphthalene (P3c) concentrations, a plot of the pseudo-first-order rate constant versus pollutant concentration is obtained. From the slope of the linear plots, a quenching rate constant value of 6.5 106 M1 s1 is determined (Figure 12).130 Quenching constants for other naphthalene derivatives such as P3ab have also been calculated from time-resolved phosphorescence experiments.182 As regards heterogeneous media, after excitation at 355 nm of phthalocyanines PC1b or PC1c immobilized in zeolites, the characteristic singlet oxygen luminiscence is monitored at 1270 nm.223 Concerning photoinduced electron transfer from a ground-state complex (pathway vi of the general mechanistic scheme), it operates in the photodegradation of methidathion (MTDT) and carbaryl (CBR) by using 2,4,6-triphenylpyrylium (TPP) and 2,4,6-triphenylthiapyrylium (TPTP) as photocatalysts (Scheme 3).162 The fluorescence of TP(T)P decreases in the presence of the pollutants, with quenching constants higher than the diffusioncontrolled rate (Figure 13). However, singlet lifetimes of TPP and TPTP are not affected by the presence of MTDT nor CBR, indicating that the quenching is not dynamic and pointing to a 1725


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Figure 11. (Top) Decay of 1O2 emission in alkaline CH3CN/D2O (4:1) recorded in the absence (black trace) and in the presence (red trace) of H1b (1.2 104 M) using RB as sensitizer and 532 nm as excitation wavelength. (Bottom) SternVolmer plot for quenching of 1 O2 by increasing concentrations of H1b, in alkaline (black) and neutral (red) media. Adapted from ref 198. Copyright 1999.

nonemissive ground-state complex as the species responsible for the reduced emission. Laser flash photolysis experiments show a less efficient formation of the triplet state of the photocatalysts upon addition of the pollutants (without a clear effect on the triplet lifetime), together with an instantaneous increase in the signal due to the pyranyl radical (Figure 14). From these experimental results an excited ground state complex has been postulated as the key active species responsible for photoinduced electron transfer. Participation of O2 3 , pathway vii,107,109,123,151,187,206 has been postulated in the photodegradation of phenol derivatives P1a, P1d, P1g, P1i, and P1j with RF, where quenching of the triplet state occurs with concomitant appearance of the RF 3 transient. A good correlation between the 3kq value and the phenol oxidation potentials is also found.123 Superoxide anion is formed by electron transfer from RF 3 to molecular oxygen regenerating the RF ground state. In this case, 1 O2 is quenched by P1a, P1d, P1g, P1i, and P1j in a physical fashion, without giving rise to oxidized photoproducts. Photocatalytic degradation of A3g in the presence of RF involves participation of 1O2, O2 3 , and H2O2 because the presence of superoxide dismutase, sodium azide, or catalase causes a delay in oxygen uptake.187 Involvement of 1O2 is monitored

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Figure 12. (Top) Typical kinetic trace of 1O2 emission monitored at 1270 nm, obtained after laser flash excitation (λexc = 355 nm) of 2-acetonaphthone in aerated solutions (CH3CN/CH2Cl2, 1:1). (Bottom) SternVolmer plot for the quenching of 1O2 by P3c, obtained from time-resolved phosphorescence experiments. Adapted from ref 130 by permission of The Royal Society of Chemistry.

by fluorescence, laser flash photolysis, and time-resolved phosphorescence experiments. In a number of examples, like photodegradation of S1c by DCA or DCAC, formation of O2 3 from P 3 after an initial photoelectron transfer process is supported by the nature of the obtained photoproducts.107,109 Photodegradation of M4 by porphyrin PP3 also occurs through an O2 3 mechanism, as indicated by the effect of benzoquinone as a superoxide scavenger and DABCO as singlet oxygen quencher.151 Finally, O2 3 is also involved in the photodegradation of atrazine (ATZ) by PP2f and PP2j, as suggested by the following observations: (a) no quenching of the triplet state by ATZ is noticed, (b) the presence of a singlet oxygen quencher does not produce any effect on the photodegradation, and (c) the triplet state of the porphyrins is efficiently quenched by O2.206 4.2. Photodegradation and Identification of Photoproducts

Photooxidation of pollutants using organic photocatalysts rarely leads to complete mineralization, or to total elimination. Instead, highly oxidized and/or fragmented compounds are obtained, which may be less toxic and more suitable for subsequent biological treatment. Therefore, the terms 1726


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Scheme 3. Proposed Mechanism for the Photodegradation of CBR by TPP and TPTP (Adapted from Ref 162; Copyright 2009)

Figure 14. (Top) 3TPP* trace obtained upon laser flash excitation (λ = 355 nm) recorded at 470 nm in the presence of increasing concentrations of CBR (03.3 104 M). (Bottom) Trace due to TPP 3 recorded at 550 nm upon addition of increasing amounts of CBR (03.3 104 M). Adapted with permission from ref 162. Copyright 2009 Elsevier BV.

Figure 13. (Top) Quenching of TPTP fluorescence (λexc = 420 nm) upon addition of increasing concentrations of CBR (03.12 103 M). (Bottom) SternVolmer plot for the quenching of TPTP fluorescence by CBR. Adapted with permission from ref 162. Copyright 2009 Elsevier BV.

photooxidation and photodegradation will be used in this section when discussing the disappearance of pollutants as a result of photocatalytic reactions, rather than removal or elimination. Different analytical procedures have been developed to evaluate the efficiency of photocatalytic processes. In some cases, classical methods such as titrations have been used; for instance, AgNO3 has found application to follow disappearance of cyanide by means of copper phthalocyanine (PC1a) supported onto zeolite X.152 Nonetheless, in most cases instrumental analytical

techniques are needed to assess photodegradation of the pollutant. Changes in the UVvisible spectrum have been employed to monitor modifications in the composition of samples during irradiation.72,154,176,183 This is a convenient procedure for studying photodegradation of different pollutants by means of riboflavin (RF) or rose bengal (RB).142,144,176,182,198,199,201 An interesting example is the photodegradation of 1,5dihydroxynaphthalene (P3c) using a porphyrin (PP2f) as photocatalyst and an iodine tungsten lamp as the irradiation source.183 The decreasing absorption in the region 275350 nm, together with the increase at λ < 275 nm (see Figure 15), is attributed to photooxidation of the pollutant, with formation of 5-hydroxy-1,4-naphthoquinone as major photoproduct (confirmed by mass spectrometry and elemental analysis). Absorbance changes have also been monitored at a selected wavelength, to obtain kinetic data130,151,183 or to compare results under different experimental conditions. For instance,183 the concentration of 1,5-dihydroxynaphthalene has been determined from 1727


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Figure 15. UV spectra obtained between 0 and 28 min during PP2f (2 104 M) catalyzed photodegradation of the naphthalene derivative P3c (2 104 M) at pH 3.8. Adapted with permission from ref 183. Copyright 2008 Elsevier BV.

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Figure 17. Degradation of aqueous solutions of ferulic acid A5d (103 M) using six different photocatalysts (10 mg L1) upon irradiation with a solar simulator. Adapted from ref 67. Copyright 2007.

Figure 18. Solar photodegradation of four phenolic acids (103 M aqueous solutions) catalyzed by TPP (red bars) and MB (green bars). Data obtained after 6 h irradiation. Adapted from ref 191. Copyright 2000.

Figure 16. Oxidation of bisphenol A (P2a) 103 M in water at pH = 12 photocatalyzed by PC3b under different flow rates of air (0200 mL min1) using visible light. Reaction monitored by HPLC using a C18 column and UVvis detection (λ = 254 nm). Adapted with permission from ref 97. Copyright 2005 Elsevier BV.

its absorption at 329 nm; with this information, the effects of initial photocatalyst concentration and pH have been established. Nevertheless, to avoid interferences due to the presence of other species absorbing at the monitoring wavelength, chromatographic analysis is often necessary. In this context, high-performance liquid chromatography (HPLC) coupled with UVvisible detection is by far the most widely employed method to follow photooxidation of pollutants.67,68,74,80,88,90,97,102,143,148,150,153,156,157,161,162,165,166, 168,170,171,173,175,177,179,180,185,186,188197,203,205,206,209,210,212,213,222 As an example, the effect of air flow rate on the photodegradation of bisphenol A (P2a) in the presence of a polynuclear zinc phthalocyanine (PC3b)97 is shown in Figure 16. Likewise, the performance of different photocatalysts has been compared by HPLC. Photooxidation of a phenolic compound, namely, ferulic acid (A5d), has been studied using a solar simulator in the presence of six different photocatalysts: 2,4,6triphenylpyrylium (TPP), 2,4,6-triphenylthiapyrylium (TPTP), acridine yellow G (AYG), methylene blue (MB), alcian blue

(PC1q), and rosolic acid (RA).67,192 Results indicate that, after 3 h of treatment, ca. 80% photodegradation of the pesticide is achieved by TPTP and AYG, ∼60% of the pollutant is photooxidized by TPP, and the other photocatalysts are less efficient (see Figure 17). A similar methodology has been employed to compare the ability of two photocatalysts (TPP and MB) to oxidize a series of phenolic acids, namely, cinnamic acid (A5a), p-coumaric acid (A5b), caffeic acid (A5c), and ferulic acid (A5d).191 Figure 18 shows that, in all cases, the electron transfer mechanism (TPP) is more efficient than singlet oxygen generation (MB). As regards the substrate, the order of reactivity observed in the TPP-catalyzed reaction is as follows: A5d > A5c > A5b > A5a. This has been attributed to the different substitution of the aromatic ring: two activating hydroxy and/or methoxy groups in A5c and A5d, only one hydroxy group in A5b, and none in the case of A5a.186 The reactivity of photocatalysts (TPP, TPTP, and AYG) toward various families of pesticides belonging to a variety of families has been evaluated.162 Methidathion (MTDT) undergoes photodegradation faster than carbaryl (CRB); TPTP and TPP are more efficient than AYG (Figure 19). Alternative chromatographic methods have also been employed: gas chromatography (GC) has been used for volatile compounds,63,109,156,158,159,161,166,184,208 whereas ionic chromatography has been mainly employed to determine ions released during the reaction.63 Both GC and HPLC have been coupled 1728


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Figure 19. Plot of the relative concentration of MTDT and CBR (C/ C0, where C0 = 50 mg L1) versus irradiation time in the presence of three different photocatalysts (10 mg L1): TPP ((), TPTP (9), and AYG (2). Filled symbols correspond to MTDT, and empty symbols correspond to CBR. Adapted with permission from ref 162. Copyright 2009 Elsevier BV.

with mass spectrometry detectors (GC-MS)83,104,105,107,109,110,116, 124,160,162,169,173,178,182,203,204,207,211 or (HPLC-MS);141,154,164 however, these hyphenated techniques have been mainly employed to detect byproduct or to elucidate the reaction mechanism, an issue that will be addressed later in this section. In a number of cases, 1H NMR105,116,173,207,211 and 13C NMR105,173,211 have been used for a more reliable identification of byproduct. Oxygen consumption (measured by means of a specific oxygen electrode) has been taken as an indicator to determine the reaction kinetics, especially for processes involving singlet oxygen as the reactive species.95,121,141,146,147,155,182,198,200202 An example is shown in Figure 20, where photooxidation of three phenolic compounds, phenol (P1a), p-phenylphenol (P1i), and p-nitrophenol (P1j), by visible light (ca. 440 nm) is catalyzed by riboflavin (RF).123 In the absence of substrate or in the dark, no oxygen is consumed; by contrast, when solutions containing RF and the substrate are irradiated, phenols with lower oxidation potentials take up oxygen at higher rates. Photophysical measurements suggest that the reaction occurs by formation of a complex between groundstate phenol and singlet oxygen, followed by electron transfer. A similar methodology has been used to compare the efficiency of a series of seven organic photocatalysts, namely, three phthalocyanines (PC1f, PC1j, and PC1l), a metal-free porphyrin (PP2e), a perylene bisimide (PBIc), rose bengal (RB), and methylene blue (MB), to achieve oxidation of phenol (P1a) (see Figure 21).167 The low activity of PC1j and PBIc has been attributed to their tendency to aggregate in aqueous solution, whereas in the case of MB, the main drawback is associated with its low photostability. Among the photocatalysts showing better performance, limitations arise from a relatively low singlet oxygen quantum yield (PC1f) or from lack of stability (PP2e and RB). Experiments carried out in the presence of detergent, at different pH values, confirm the influence of aggregation and stability on the reaction rate.224 In some cases, the effect of operational conditions on the photochemical process has been studied using statistics and, in particular, response surface methodologies. An example can be found in Figure 22, where the effect of photocatalyst (TPP) and substrate concentration (xylidin, M3f) on the photooxidation is illustrated by means of an experimental design methodology

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Figure 20. Oxygen consumption measured upon visible light (440 nm) irradiation of methanol/water solutions containing RF (2 102 M) and three phenols (1 101 M): P1a (b), P1i (9), and P1j (2). Adapted with permission from ref 123. Copyright 2004 Elsevier BV.

Figure 21. Oxygen uptake rate measured upon irradiation of aqueous solutions (pH = 13) containing phenol (P1a, 7.16 103 M) and seven different photocatalysts (5 106 M). Adapted with permission from ref 167. Copyright 1997 Elsevier BV.

based on Doehlert matrixes.212 Figure 22 shows that higher percentages of photodegradation are achieved at lower substrate concentrations and in the presence of higher amounts of TPP. In addition to monitoring photodegradation by chromatographic methods, in a limited number of cases the photoproducts have been identified by comparison with standards and/or by NMR. However, further effort is required to investigate the nature of the reaction products obtained by treatment of pollutants with organic photocatalysts, to elucidate the involved mechanism. For example, different photocatalysts have been used in the photooxidation of a wide variety of model sulfides. Thus, sulfides S1a, S1c, and S1e are converted into the corresponding sulfoxides, sulfones, and disulfides by photocatalytic treatment with DCA, AQ, or MB.63,104107,109,158 Under similar conditions, disulfides (S2ab) give thiosulfonates and sulfonic acids as the major photoproducts.105,109 Interestingly, photooxidation of sulfur-containing compounds may depend on the nature of the photocatalyst and also on the presence or absence of O2.156,157,161 Thus, irradiation of S1j in the presence of DCA or TPP gives rise to the sulfoxide as the main photoproduct. It is formed either by reaction with 1O2 (generated from DCA) or by 1729


Chemical Reviews electron transfer quenching of the DCA/TPP excited states, followed by reaction of the resulting radical ions with O2. Alternatively, deprotonation of the sulfide radical cation can give rise to radical coupling products or, after oxygen trapping, to benzaldehyde (Scheme 4).161 One of the most studied families of pollutants or model compounds as targets for organic photocatalysis are phenols and their derivatives. Thus, P1a is oxidized to p-benzoquinone using porphyrin PP2f172 or different phthalocyanines such as PC1f, PC1g, or PC1l.148,167,172 The same product is obtained when P1g is photooxidized with porphyrins PP2jm88,150 or phthalocyanines PC1g, PC1l, or PC1op94,148 or when quinol (P1c) is treated with RB or MB.124 Catechol (P1b) and tyrosol (P1h) give dimers as the major photodegradation products with RB as photocatalyst.173 Photodegradation of P1j with PC1l or PC1p gives 4-nitrocatechol and hydroquinone,154 whereas 2,6-dimethylphenol (P1l) is transformed into 2,6-dimethylbenzoquinone using porphyrins PP2j, PP2l, or PP2m.88 Chlorinated phenols such as 2,4,5-trichlorophenol (P1n) or

Figure 22. Response surface obtained for the percent of xylidine (M3f) photodegradation in aqueous solution (pH = 3) after 60 min of irradiation at different concentrations of catalyst and substrate, using a Doehlert matrix. Adapted from ref 212. Copyright 2004.

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pentachlorophenol (P1p) give 2,5-dichloro-1,4-benzoquinone and 2,3,5,6-tetrachloro-1,4-benzoquinone, respectively, by photocatalytic treatment with RB, MB, or PC1g.178,179 Rose bengal mediates photodegradation of 2-chlorophenol (P1e) to pyrocatechol, 2-chloro-1,4-benzoquinone, 2-chlorohydroquinone, and maleic acid.80 Naphthalene derivatives, such as 1-naphthol (P3a), afford 1,4-naphthoquinone as the major photoproduct when irradiated in the presence of MB or RB,124,182 whereas in the case of P3b, phthalic acid and a dimer (2,20 -dihydroxy-1,10 -binaphthyl) are formed.182 Dihydroxynaphthalenes like P3c undergo photodegradation by means of PP2f or PP2i, leading to 5-hydroxy-1,4naphthoquinone as the major photoproduct.130,183 Oxidative degradation of bisphenol derivatives (P2ac) has been achieved using RB, RF, MB, or PC3ab as photocatalysts.97,141,181 For example, in the case of P2a the photoreaction leads to p-isopropenylphenol, p-isopropylphenol, hydroquinone, and phenol, in a first stage. Further oxidation gives oxalic and maleic acids and, ultimately, carbon dioxide (Scheme 5). Likewise, photochemical treatment of P1h or A2 with TPP proceeds through initial oxidation of the benzylic position, to give in both cases p-hydroxybenzaldehyde.185 Under similar conditions, p-coumaric acid (A5b) gives also p-hydroxybenzaldehyde, together with p-hydroxybenzoic acid, protocathechuic aldehyde, and maleic and oxalic acids.193 Under RB photocatalysis, heteroaromatic compounds, for instance, H1a, are converted into maleimide, succinimide, 2,3-dihydroxypyridine, and maleic, fumaric, and oxalic acids.144,198 By contrast, isomerization rather than oxidation occurs with E-cinnamic acid (A5a) in the presence of perylene bisimide PBIa, giving the Z isomer as major photoproduct.83 The dichlorinated pesticide M1 as well as carbamates C1a and C1c have been treated with AQ as photocatalyst. 110 Thus, M1 leads mainly to 1,1-dichloro-2,2-bis(4-acetylphenyl)ethane, whereas C1a yields 2-hydroxybenzaldehyde and 3-methylbenzo[e][1,3]oxazine-2,4-dione, and C1c gives rise to 3,5-dimethyl-4methylsulfinylphenol and 3,5-dimethyl-4-methylthiophenol. On the other hand, carbamate C1b affords o-dihydroxybenzene as the major photoproduct when photodegraded with TPP in heterogeneous media.196 In particular, photooxidation of PTE by anthracene (ANT) on a polymeric support yields paraoxon ethyl and 4-nitrophenol as the primary products102 (Scheme 6). Likewise, irradiation of

Scheme 4. Mechanistic Pathways in the Photooxidation of Sulfide S1j Sensitized by DCA and TPP (Adapted with Permission from Ref 161. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA)

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Chemical Reviews PTM in the presence of heterogeneous TPP results in formation of methyl paraoxon and 4-nitrophenol.163,164 Photochemical decomposition of TDM in the presence of DCA or TPP results in oxidation of the hydroxyl group or in fragmentation (Scheme 7).207 ATZ and the related compounds ATT and AMT are photodegraded by RF,203,204 porphyrins PP2f or PP2j,206 and phthalocyanine PC1i.205 Oxidation or cleavage of the lateral chains gives rise to a mixture of photoproducts (Scheme 8). Further examples are the photodegradation of TNT by PP2f, PP2g, or PP2h, to give trinitrobenzoic acid and trinitrobenzene;210 the dehalogenation of DDT using MG as photocatalyst;211 the oxidation of M5 with RF, leading to 1,6-benzo[a]pyrenedione, 3,6benzo[a]pyrenedione, and 6,12-benzo[a]pyrenedione;214,215 the photodecarboxylation of M6 in the presence of supported TPP;216 the oxygenation of the aromatic ring of M7 in the presence of RB;217 or the singlet oxygenation of the neonicotinoid insecticides M8 and M9ab, affording 6-chloronicotinic acid.129

5. PHOTOCATALYTIC APPLICATIONS 5.1. Organic Photocatalysis in Heterogeneous Media

5.1.1. Preparation and Characterization of Supported Organic Photocatalysts. After establishing the feasibility of using organic dyes as photocatalysts, hetereogeneization of the systems seems a logical step forward; solid photocatalysts can be Scheme 5. Oxidative Degradation of Bisphenol A (Adapted with Permission from Ref 97. Copyright 2005 Elsevier BV)

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easily removed after the reaction, making it possible to operate in a continuous mode and thus to recycle the photocatalyst for further use. For this purpose, some strategies have been used to support catalysts onto different materials. By far, 2,4,6-triphenylpyrylium (TPP) and 2,4,6-triphenylthiapyrylium (TPTP) hosted onto different inorganic supports are the most widely employed solid photocatalysts for elimination of model pollutants from aqueous solution. In some cases silica gel157,195 or naturally occurring silicates, such as sepiolites, have been used;194 in addition, TPP has been hosted onto carbon nanotubes225 or mesoporous titanium dioxide.226 Nonetheless, in most cases, zeolites have been employed as supports following different experimental procedures for incorporation of TP(T)P. Thus, TPP has been included within extralarge pore zeolitic aluminosilicates, such as MCM-41. These materials provide an adequate balance between moderate cage effect and facilitation of molecular traffic through the mesopores.227 As the diameter of the channels (2 nm) allows diffusion of TPP, a simple ionexchange procedure can be employed to incorporate this organic cation (Figure 23). Easily available Y-zeolite has also been employed as host for (thia)pyrylium cations. The dimensions of this material are compatible with the presence of large organic cations (0.95 nm 1.2 nm) inside its supercages (1.3 nm diameter), yet the connecting channels are too small for free diffusion (0.74 nm). Hence, more complex processes have been developed to synthesize the desired hybrid materials, for instance, the “ship in a bottle” methodology. This procedure has been followed to synthesize different photocatalysts inside the cages of the zeolites from their immediate precursors, which are small enough to diffuse through the channels.228 In the case of TPP, chalcone and acetophenone are mixed with acid Y-zeolite (HY) as illustrated in Figure 24; the Br€onsted acid sites of the zeolite play a catalytic role.229 The presence of TPP in the new material, as well as the absence of its precursors after thorough washing, can be checked by Fourier transform infrared (FT-IR), UV-diffuse reflectance, Scheme 8. Photocatalyzed Reactions of Triazine Derivatives (Adapted with Permission from Ref 203. Copyright 2002 Elsevier BV)

Scheme 6. Photocatalytic Oxidation of PTE (Adapted with Permission from Ref 102. Copyright 2005 Elsevier BV)

Scheme 7. Photodegradation of TDM in the Presence of DCA or TPP (Adapted from Ref 207; Copyright 2003)

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Chemical Reviews and temperature-programmed desorption (TPD) analysis. There are some reports of photocatalytic applications for the oxidation of hazardous chemicals, using TPP-loaded Y-zeolite prepared by this procedure.188,216 Likewise, introduction of TPTP159and bipyrylium (BP)68 inside Y-zeolite is achieved by an analogous strategy. It is interesting that, in the case of BP, the photocatalyst seems to occupy two neighboring supercages. More recently, the “camel through the eye of a needle” approach has been proposed as an alternative to imprison TPP inside the Y-zeolite supercages. It is formally an ion-exchange process in aqueous medium, although it actually follows a more complex mechanism, summarized in Figure 25: (a) hydrolytic opening of the pyrylium ion to give a linear diketone, (b) diffusion of this diketone through the channel to reach the supercage, and (c) recyclization to TPP upon heating.213 This mechanism is supported by the IR spectrum of the final product, which shows the characteristic bands of TPP. In addition, a sample submitted to partial dehydration shows the presence of the diketone together with residual TPP (see Figure 26). This methodology allows for a better control of zeolite loading, which can be tuned in the range 315% (w/w). 189 Modifications of the above procedure make use of 1,3,5-triphenylpent-2-en-1,5-dione as starting reagent; it is incorporated into the zeolite by stirring in an organic solvent, followed by filtration and final dehydration. This approach has been extended to host

Figure 23. Incorporation of TPP inside the channels of mesoporous MCM-41. Adapted with permission from ref 227. Copyright 1994 American Chemical Society.

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TPP in Y and β zeolite,218 MCM-41, mesoporous TiO2SiO2, and SiO2.230 Supercritical CO2 at 60 °C can be used instead of the organic solvent for impregnation of Y-zeolite with 1,3,5-triphenylpent-2-en-1,5-dione; at higher temperature, cyclization of the diketone occurs inside the zeolite supercages to give TPP.231 The resulting material has been characterized by X-ray diffraction, FTIR, UV diffuse reflectance, and N2 adsorption at 77 K. The “ship in a bottle” procedure has also been used with Y zeolites to host Fe(II), Mn(II), Co(II), Cu(II), Zn(II), and Ni(II) phthalocyanines. First, ion exchange is employed to load Na zeolite with the desired metal, and then o-phthalonitrile is added to synthesize “in situ” the organic moiety.230 More simple is the introduction of methylene blue (MB) inside the cavities of Y zeolites, which can be achieved by direct ion exchange.158 A series of papers have appeared dealing with the use of silica gel to support different photocatalysts,6266,105107,172 to remove gas-phase as well as dissolved pollutants. A very simple procedure has been described to host 9,10-dicyanoanthracene (DCA) and anthraquinone (AQ) onto large-particle silica gel, in which the support is added to a solution of the catalyst and then filtered and dried.106 For obtaining porous silica monoliths containing DCA, tetramethyl-ortho-silicate is used as starting material, and the synthesis is carried out “in situ” in a solution containing DCA. Characterization of the material is achieved by N2 adsorption at 77 K and diffuse reflectance UV. Alternatively, DCA is first grafted onto triethoxysilyl precursor to synthesize silica particles afterward.107,232,233 Other photocatalysts that have been supported onto silica include ketones,230 condensed aromatics,231 tin porphyrins,90 and the above-mentioned pyrylium salts.157,195 In a related application, formation of a silane gel from its precursors in the presence of photocatalysts has been employed to immobilize rose bengal (RB) and methylene blue (MB).143,175 Supported photocatalysts have also been prepared from alumina, which has been used for different phthalocyanines.146 Likewise, bentonite has found application as host for phthalocyanines and MB,98,124,171,234 whereas MgAl layered double hydroxides have been used to host metal phthalocyanines180 as well as 4-benzoylbenzoate.235,236 Biodegradable organic polymers, such as starch, dextran, chitosan, or hydroxyethylcellulose, can be used to support organic photocatalysts for wastewater treatment.102,237242 This constitutes a novel application of polymer-based photosensitizers, which were introduced originally for photo-oxidations.243 For instance, an anthracene (ANT)-substituted dextran has been prepared by etherification with 9-chloromethylanthracene. The obtained

Figure 24. Loading of Y-zeolite with TPP following a “ship in a bottle” synthesis using acetophenone and chalcone as precursors. Adapted with permission from ref 229. Copyright 1994 American Chemical Society. 1732


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Figure 25. Mechanism of formal ion exchange to introduce TPP inside the Y-zeolite: (a) hydrolytic ring-opening leading to a linear diketone, (b) diffusion through the channel, and (c) thermal recyclization inside the supercage. Adapted with permission from ref 213. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 26. Infrared spectra of (A) TPP in KBr, (B) TPP hosted in Y-zeolite (dry product), (C) TPP hosted in Y-zeolite (wet product), and (D) 1,3,5-triphenylpent-2-en-1,5-dione. Adapted with permission from ref 213. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA.

polymer has been characterized by 1H NMR, FT-IR, emission, and UVvis absorption spectroscopy, as well as by gel permeation chromatography (GPC).238 A modified chitosan has been obtained by functionalization with 1-naphthylacetic acid.241 In this case, dynamic light scattering and atomic force microscopy have been used, in addition to spectroscopic methods and GPC, for characterization of the polymer. Resins are alternative supports for photocatalysts. In particular, porphyrin PP1 has been supported onto Amberlite, which is an ion-exchange resin.205 Amberlite as support for photosensitizers has been used for other applications.244 Different ion exchangers (Amberlite, Dowex) have been used to support a series of phthalocyanines and porphyrins to check their photostability and

performance using phenol as model compound.172 Finally polymeric ion exchangers have been attached to RB.169,245 In all cases the procedure is simple, basically stirring the ion exchanger and the photocatalyst in an appropriate medium. 5.1.2. Photocatalysts Stability and Reuse. One of the major problems associated with the use of organic photocatalysts is their limited stability, as they can suffer photobleaching or solvolytic attack in the reaction medium. An example of this problem is the case of TPP in aqueous solutions.195 This observation is explained by hydrolytic ring-opening of the heterocycle, outlined in Scheme 9. Pseudo-first-order rate constants have been determined for bleaching of TPP at different pH values, both under irradiation (solar simulator) and in the dark. Data in Figure 27 indicate that TPP is moderately stable only at pH < 3 (useless for practical applications) and that degradation is accelerated under irradiation.195 As a consequence of the limited TPP stability, its efficiency decreases along the course of the reaction. By contrast, when this organic cation is supported onto silica gel plates, complete elimination of ferulic acid (A5d) is achieved without any significant loss of efficiency after 6 h of reaction. Interestingly, upon incorporation of TPP inside the supercages of Y zeolite, the hybrid material does not show any changes in the intensity of the characteristic UVvis TPP bands when it is stirred in water;163 actually the encapsulated cation is indefinitely stable even at neutral pH. This stabilization has been attributed to geometrical constraints imposed by the rigid zeolite framework that make attack by reactive species to the encaged pyrylium ring more difficult.218 Conversely, some hydrolysis occurs within MCM-41 zeolite or onto silica gel, where the steric confinement is less marked.246 1733


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Scheme 9. Hydrolytic Opening of the Pyrylium Ring to Give a Nonactive Diketone (Adapted with Permission from Ref 195. Copyright 2002 Elsevier BV)

Figure 28. Photodegradation of xylidine M3f (100 mg L1) in aqueous solution, at pH = 2, photocatalyzed by TPP, supported onto Y zeolite (1.2 g L1). Adapted with permission from ref 213. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 27. Pseudo-first-order rate constants for TPP degradation in aqueous medium (10 mg L1) at different pH values. Adapted with permission from ref 195. Copyright 2002 Elsevier BV.

Nonetheless, heterogeneization does not always guarantee an enhanced stability of the photocatalyst. For instance, irradiation of anthracene substituted dextran results in an important photodegradation,102 as indicated by the strong inhibition of fluorescence with time. This has been attributed to oxidation of the anthracene moiety, to give a nonemitting anthraquinone derivative. In a similar process with naphthalene-modified hydroxyethylcellulose, not only is the photocatalyst is oxidized but degradation of the polymer is also observed.240 An additional advantage of heterogeneization is the possibility of recycling the photocatalysts for further use. Thus, TPP supported onto Y zeolite has been employed for abatement of xylidine (M3f) in aqueous solution (Figure 28). After elimination of the pollutant, an extra amount of M3f is added for a new photocatalytic cycle. Figure 29 shows a logarithmic plot of the remaining concentration of M3f versus time for three cycles; the parallel lines indicate that efficiency is maintained, as similar pseudo-first-order rate constants are obtained. In a different report, supported TPP has been used for the oxidation of organic sulfides, without significant loss of activity.157 Only when TPP is adsorbed onto silica gel does the efficiency decrease, an effect that has been attributed to leaching of the organic catalyst. In another example,98 photooxidation of trichlorophenol (P1o) is catalyzed by a Pd-phthalocyaninesulfonate (PC1k) supported onto a modified bentonite. Figure 30 shows that, although the efficiency decreases slightly with the number of cycles, seven consecutive runs can be carried out with complete elimination of the pollutant. The gradual deactivation of the photocatalyst has

Figure 29. Photocatalytic abatement of M3f (C0 = 100 mg L1) in aqueous solution, at pH = 2, by TPP (C0 = 1.2 g L1) on Y zeolite. Repeated cycles with the same batch of catalyst result in similar rate constants. Adapted with permission from ref 213. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA.

been attributed to adsorption of the formed intermediates onto the bentonite surface, together with some decrease in the catalyst concentration because of sampling. A diverging result has been obtained143 when using RB impregnated in a silane gel, deposited onto glass plates, to photooxidize 2-chlorophenol (P1e). Figure 31 shows that the percentage of pollutant photodegradation decreases with reuse, to reach stable values after 34 cycles. Overall, these facts seem promising as a proof of the concept; however, for practical applications, determination of the “turnover number” and investigation of the possible poisoning phenomena is a necessary step forward. 5.1.3. Efficiency. The improvements achieved in the synthesis and stabilization of supported organic photocatalysts have prompted their application to remove pollutants as well as the development of further research for optimization of operational variables.143,160,164,166,171,175,180,196,212,218,230 As an example, abatement of phenol under visible light irradiation (halogen lamp) has been performed in the presence of an aluminum phthalocyanine, PC1f.171 Different operational parameters have been studied, such as the sorption of phenol, the effect of photocatalyst loading, or the possible recycling. In particular, PC1f has been compared with CoCu containing phthalocyanines. The aluminum-based photocatalyst is more efficient than the copper or cobalt analogues. This is explained by considering that singlet oxygen is the main oxidizing agent; hence, the presence of paramagnetic transition metals, such as Cu or Co, 1734


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Figure 30. Recycling experiment using PC1k (0.05 g L1) onto a modified bentonite (1.0% w/w) for the phenolic pollutant P1o degradation in a mixture of water and dimethylformamide (3:2, v/v) under visible light. The photocatalyst remains stable after seven repeated runs. Adapted with permission from ref 98. Copyright 2005 American Chemical Society.

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Figure 32. Degradation of a variety of substituted phenols (in aqueous solutions at pH = 12) using phthalocyanine PC1f on a modified bentonite as heterogeneus photocatalyst (0.25% w/w). Adapted with permission from ref 171. Copyright 2005 American Chemical Society.

Figure 33. Degradation of chlorophenol P1o (3.1 104 M in water at pH = 6) using homogeneous PC1k (8 103 g L1) or different heterogeneous photocatalysts based on this phthalocyanine (1 g L1). Adapted with permission from ref 180. Copyright 2007 American Chemical Society. Figure 31. Efficiency of P1e (C0 = 5 104 M) degradation in phosphoric buffer (pH = 7), after repeated experiments using RB impregnated in a silane gel as photocatalyst. Adapted with permission from ref 143. Copyright 2004 Elsevier BV.

results in shorter triplet lifetimes, and therefore triplet quenching with formation of singlet oxygen is less efficient. The effect of the substrate has also been studied. Figure 32 shows photodegradation of phenol (P1a), 4-chlorophenol (P1g), 4-nitrophenol (P1j), 2,4-dichlorophenol (P1k), and 2,4,6-trichlorophenol (P1o). Because singlet oxygen reacts more efficiently with electron-rich substrates, the reaction is markedly slower in the case of P1f bearing a deactivating group. A specific factor to be taken into account in heterogeneous photocatalysis is the effect of support. Figure 33 shows the photooxidation, under visible light irradiation, of trichlorophenol (P1o) catalyzed by a palladium phthalocyanine (PC1k) hosted in different related supports.180 The support itself, a layered double hydroxide (LDH), is inactive, as shown by a blank experiment. Heterogeneized PC1k is more efficient than the homogeneous photocatalyst. This is attributed to the aggregation of dissolved PC1k molecules, which results in self-quenching of the excited dye and thereby in a loss of efficiency in the formation of singlet oxygen; this phenomenon is reduced by means of heterogeneous catalysis. In addition, adsorption of the pollutant on the support brings the catalyst and the substrate in close proximity, so that singlet oxygen does not need to

diffuse far away from where it is produced. Differences among supports are also accounted for in terms of dye aggregation. The performance of TPP and TPTP onto Y and β zeolites has also been probed165,166 using aniline (M3a) and phenol (P1a) as model compounds. The higher efficiency of TPTP-based materials (see, e.g., Figure 34) has been attributed to the increased oxidizing abilitity of TPTP. The photocatalytic activity of different phthalocyanines (PC1b and PC1c) and TPP supported onto SiO2, TiO2SiO2, and zeolites has been compared using yperite (S1g) as pollutant.160 Data in Figure 35 indicate that Y zeolite-based photocatalyst is very efficient despite the limitation associated with diffusion of the contaminant through the channels; this is attributed to a cooperative effect of the host, adsorbing yperite and favoring the contact between catalyst and substrate. A comparison between UV and ambient light has also been made in the same work. Under UV irradiation, PC1b is more efficient than PC1c, while the reverse is true for ambient light; this is explained as the result of the different absorption spectra of the two photocatalysts. The high efficiency of TPP hosted in Y zeolites under ambient light is remarkable. Recently, a more quantitative approach to explain the effect of the support230 has been provided by calculating the band gap for SiO2, TiO2SiO2 MCM-41, and Y zeolite loaded with TPP, PC1b, and PC1c. Thus, photooxidation of dipropyl sulfide is achieved with materials showing lower band gap energies. 1735


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Figure 34. Photodegradation of aniline (M3a) in aqueous solutions (40 mg L1) in the presence of various heterogeneous photocatalysts (1.5 g L1): (a) TPP@Y, (b) TPTP@Y, and (c) TPTP@β. Adapted from ref 165 by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC.

Figure 36. (Top) Concentration of PTM versus irradiation time: (a) in the absence of photocatalyst, (b) in the presence of TPPSiO2 (7.5 g L1), or (c) in the presence of TiO2 (2 g L1) upon direct exposure to sunlight. (Bottom) Concentration of PTM versus irradiation time in the presence of TPP supported onto Y zeolite (7.5 g L1) measured in the aqueous phase (d) after exhaustive extraction of the pollutant and (e) during the irradiation. Adapted with permission from ref 164. Copyright 2000 Elsevier BV. Figure 35. Effect of the support employed with three photocatalysts (PC1b, PC1c, and TPP, 0.34 g L1) to remove yperite (S1g) in dichloromethane solution (170 ppm). Percentages of photoabatement after 1 and 2 h of UV irradiation in an aerated reactor. Adapted with permission from ref 160. Copyright 2008 American Chemical Society.

Because TiO2 is the most widely used heterogeneous photocatalyst for wastewater treatment, it seems meaningful to compare the performance of heterogeneized organic catalysts with that of TiO2. An example is provided by the photodegradation of methyl parathion (PTM).164 Figure 36 indicates very efficient photodegradation of PTM achieved by TiO2, faster than by TPP supported onto SiO2 and Y zeolite. In the last case, fast absorption of PTM in the zeolite channels is followed by a slower oxidation of this pesticide inside the supercages; this is in agreement with detection of higher amounts of PTM when zeolites are submitted to thorough extraction. Other operational parameters have also been optimized in heterogeneous photocatalysis. They include catalyst concentration (TPP hosted in Y zeolite) and initial substrate concentration (xylidine, M3f).212 Higher initial concentrations of M3f result in a lower pseudo-first-order constant of pollutant consumption (k). The catalyst amount is important below 1.5 g L1; beyond this point, addition of extra photocatalyst results in no significant increase of k. This can be easily appreciated in Figure 37, where

the k values obtained with a xylidine concentration of 100 mg L1 are plotted against catalyst concentration. When the amount of photocatalyst is higher than 1.21.6 g L1, the rate constant reaches a plateau. This is well-known in heterogeneous photocatalysis: once the catalyst amount absorbs all incident photons, a further increase does not lead to a further enhancement of the reaction rate. 5.1.4. Organic Versus Inorganic Photocatalysts. To assess the applicability of organic photocatalysts for the decontamination of real wastewaters, it seems interesting to compare them to inorganic photocatalysts, mainly TiO2, in terms of their intrinsic properties and/or their reactivity in the photooxidation of pollutants and model compounds. A first issue is the fraction of solar light that can be used to activate the process. In this context, it is widely accepted that titanium dioxide exhibits high efficiency in combination with ultraviolet irradiation. However, a logical step forward would be to take advantage of a broader spectral range by including visible light. Here, organic photocatalysts may provide a complementary tool, as their absorption bands (see Figure 1) may extend to the near-infrared.218 By contrast, one of the main drawbacks of organic photocataysts is that they are less robust than TiO2. For instance, upon irradiation in aqueous medium, the pyrylium cation (TPP) undergoes rapid hydrolytic ring-opening,218 giving rise to 1,3,5-triphenyl-2-penten-1,5-dione 1736


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Figure 37. Heterogeneous photocatalysis for the oxidation of M3f (100 mg L1 in aqueous media at pH = 3) by TPP-containing zeolite: plot of the pseudo-first-order constant k (min1) versus the amount of heterogeneous catalyst (C0 = 100 mg L1). Adapted with permission from ref 212. Copyright 2004 Elsevier BV.

(Scheme 9).195 Fortunately, this disadvantage can be overcome by adsorption onto inorganic supports.166 Thus, when TPP is encapsulated within zeolites Y or β, it can persist indefinitely, because the geometry of the transition state required for attack by the hydroxyl group does not accommodate within the rigid crystalline framework.218 This effect is also observed, albeit to a lesser extent, with extra-large-pore MCM-41 or amorphous SiO2.166 In view of these limitations, the activity of organic photocatalysts has been compared to that of TiO 2 mainly when the former are stabilized by incorporation to inorganic supports.62,63,159,163,165,166,188,196,218 In general, the higher surface area of zeolite-based photocatalysts justifies their higher activity. This is the case of TPP or its thia analogue TPTP encapsulated within zeolites Y or β, which exhibit higher intrinsic activity than TiO2 in the photooxidation of thianthrene S6159,166 or p-chlorophenoxyacetic acid (A4a).188 This trend is even more remarkable when the effectiveness per photocatalytic site is considered. As an example, TPP at β shows a kinetic rate constant 1 order of magnitude higher than that of TiO2 in the photooxidation of phenol P1k.218 However, in some cases heterogeneous organic photocatalysts are similar to or even less photoactive than P-25 TiO2. Thus, TPP at zeolite Y shows similar photocatalytic efficiency as TiO2 for the oxidation of methylparathion (PTM)163 whereas TiO2 is superior for the photodegradation of phenol (P1a)165 or 2,4-dichlorophenol (P1k).218 Interestingly, the trends observed with the initial activities are not always coincident with those found for the final conversions of a pollutant. Thus, they are parallel for phenols P1a or P1k165,218 whereas they converge to similar values when the reactivities of S6 with P-25 TiO2 and TPP or TPTP at either Y or β are compared.159 This can be associated with a variety of phenomena, such as progressive blocking of the zeolite pores, poisoning, interference of photoproducts, etc. In addition to the overall photocatalytic efficiency, a technical problem from the practical point of view is the poor dispersibility of supported organic photocatalysts, which is worse than that of TiO2 and has to be improved. Thus, TPP on β zeolite displays larger particle size (>0.8 μm) than the nanometric TiO2 (ca. 300500 nm), which results in sedimentation of the former.218 Mineralization is a widely used endpoint in wastewater treatment. In this connection, the use of TiO2 generally results in higher TOC removal, compared to organic photocatalysts. This is observed in the photodegradation of P1k by TiO2 or TPP at β zeolite.218 However, when mineralization is not complete, a

REVIEW

number of photoproducts with different oxidative states can be formed. Depending on their toxicities, it may be appropriate to combine the photocatalytic step with a subsequent biological treatment to match the desired decontamination levels. In this context, an important aspect that has to be taken into account is the reaction mechanism, because it determines the nature of the subsequent obtained photoproducts. As an example, TiO2 operates through an electron-transfer mechanism, whereas dicyanoanthracene (DCA) generates singlet oxygen as the main reactive species.62,63 Accordingly, the main photoproduct in the TiO2-mediated photodegradation of dimethylsulfide S1a is dimethyldisulfide whereas photooxidation with DCA on silica monolites leads to dimethylsulfoxide and dimethylsulfone.62 At the present stage, it is too early to anticipate the potential of organic photocatalysts to compete with TiO2 for dealing with environmental problems in real life. This would require evaluation of their performance in pilot plants and water treatment facilities, taking also into account factors such as engineering or cost efficiency. However, if the above-mentioned problems (stability, dispersibility, mineralization, etc.) are satisfactorily addressed in future investigations, the prospects for application seem promising. 5.2. Environmental Applicability: Use of Solar Light, Scaleup, Detoxification, and Biodegradability

As indicated in the introduction, the use of sunlight in photocatalysis is an interesting approach, as it might result in an enhanced sustainability of the process.247 Because the absorption spectra of many organic photocatalysts show important bands in the UVAvisible domain (see Figure 1), their use under real solar irradiation seems to be a logical step forward. The use of UVvis emitting lamps90,97,107,159,163,164,173,188,205,207,222 or commercial solar simulators67,74,143,185,189,192,193,195,204,205,218 are interesting as their emission spectra closely match the solar light, and they are not subjected to the intrinsically variable conditions of real exposure to sun; hence, they are appropriate to obtain kinetic data, to compare different photocatalysts or to determine the effect of operational parameters (catalyst amount, pH, pollutant concentration...) on the results. Information is also available on the use of real sunlight for elimination of pollutants. In some cases, very simple experimental set-ups have been used, consisting in flasks or glass vessels left stand in sunny places.67,74,107,143,164,185,186,188,189,191,193195, 203,204,207,209 A more complex system is based on a plate reactor, able to work in semicontinuous mode (see Figure 38).143 The flow rate is adjusted to ensure exchange of the total volume (450 mL) every 9 min. Parallel experiments have been performed using either artificial lamps or real sunlight.107,164,185,188,189,193,204,207 As an example, Figure 39 shows the results obtained by comparing tungsten lamps with solar irradiation using bisphenol A (P2a) as model pollutant and PC3b as photocatalyst.97 As expected, higher lamp potencies result in faster degradation of the pollutant. Although experimental conditions are not the same, and hence comparisons cannot be made straightforward, rather promising results are obtained when real sunlight is employed (complete photodegradation of the pollutant after 40 min). Recent work has explored the possibilities of solar-based decontamination processes involving organic photocatalysts, which have been scaled-up to gain some insight into the real applicability of these techniques. Plants based on compound parabolic collectors (CPCs) are the most commonly used in solar water treatment (photo-Fenton or photocatalytic titanium 1737


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Figure 38. Schematic experimental setup for solar-based experiments consisting of a solar reactor, peristaltic pump, homogeneization/aeration vessel, gas inlet, sampling port, and magnetic stirrer. Adapted with permission from ref 143. Copyright 2004 Elsevier BV.

3

Figure 39. Degradation of bisphenol A (10 M in aqueous media at pH = 12) (monitored at λ = 254 nm) using PC3b as photocatalyst (2 104 M), employing sunlight or different tungsten lamps. Adapted with permission from ref 97. Copyright 2005 Elsevier BV.

dioxide).247 This kind of plants consists in an array of pyrex tubes connected in series, through which the effluent flows. Two parabolic mirrors, generally made of aluminum, concentrate solar radiation in the pyrex tube. The setup generally includes a radiometer to measure the instantaneous and accumulated solar irradiation. This configuration allows the use of both, direct and diffuse solar light; additional practical advantages are the relative low price, robustness and simplicity.30 An example of this type of plant can be seen in Figure 40. However, other plants have been employed with a configuration allowing a concentration of 20.9 suns,83 which is able to rotate automatically in order to optimize the angle of incidence. Photodegradation of ferulic acid (A5d) photocatalyzed by AYG has been scaled-up using a CPC-based pilot plant of 4 L capacity.74 Figure 41 shows the variation of different parameters monitored along the solar process, namely pollutant concentration and total organic carbon (TOC). The time scale is represented as t30W. This is a convenient form to normalize the intrinsically variable incident radiation during the experiment: the accumulated radiation is calculated, and then converted into time by considering an average UVA irradiance of 30 W/m2.248 As an indicator of solution oxidation state carbon oxidation state (COS) is also represented.249

Figure 40. Pilot plant for photocatalytic treatment based on CPCs technology.

Figure 41. Photodegradation of ferulic acid (A5d, 103 M) catalyzed by AYG (10 mg L1) under solar irradiation in a 4 L CPC-based pilot plant. Left y-axis, given in relative units, (() ferulic acid concentration and (9) TOC. Right y-axis, COS (). Adapted with permission from ref 74. Copyright 2007 Elsevier BV.

Figure 42. Toxicity measured through inhibition of the respiration of activated sludge for 50 mg L1 aqueous solutions of methidathion (MTDT) (blue bars) and carbaryl (CBR) (red bars) before and after 3 h of irradiation in a solar simulator, in the presence of three different photocatalysts (AYG, TPP, and TPTP, 10 mg L1). Adapted from ref 162. Copyright 2009.

Although nearly 80% pollutant degradation is achieved after 5 h of irradiation, mineralization is negligible, which is attributed to formation of organic intermediates more reluctant to photooxidation than the parent compound. Accordingly, COS values are stabilized 1738


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Table 5. Types of Studies Performed on the Photooxidation of Different Pollutants with Organic Photocatalysts mechanistic evidence TPP

S1b156 109

S1c

S1b156,157

S1b156,157

109

109

S1c

156

S1f

S1f

161

S1j

S1h

161

157

S1j

157,161

S1j

157,161

S1g

S1h

S1i

S1i

160 157

161

161

161

S1f

S1h

157

S1b157 S1f

107,156,157,159 157

S1g

161

photocatalysts 157,159

S1c

107,156,157,159 160

S1i

S1k

heterogeneous photodegradation photoproducts

161

159

S2c

159

S1l S1m161

S1j S1k161

S1k S1l161

S4 S5159

S3116

S1l161

S1m161

S6159

116

161

S7

159

S1m 162

MTDT

163,164

S2c

PTM

116

68,165,166

S3

P1a

159

S1f

157

S1h

157

157

S1j

PTM

163,164

166

P1a

environmental

photocatalysts

applicability

S1b156,157

MTDT162

109

PTM163,164

156,157,159

P1h185

160

P1k218

157

A2185

S1c S1f

S1g

S1h

161

A3a186

157,161

S1i

P1h P1k218

S1j S1k161

A3c186 A3d186

A2185

S1l161

A3e186

195

A5d

M3a

166

S1m

159

S2c

116

A3f186 A3g186

S3

S4

P1k

S3

A3h186

S4159

S5159

A4a188

S4159

A4a188

A2185

S5159

S6159

A5a189

S5159

A5a189191

A3a186 A3c186

S6159 S7116

S7116 MTDT162

A5b189 A5c189

S6159 S7116

A5b189191,193 A5c189192

A3d186

MTDT162

PTM163,164

A5d189,194,195

MTDT162

A5d67,189192,194,195

A3e186

PTM163,164

P1b173

C1b196

PTM164

CBR67,162

A3f186

P1a68,165,166

P1h173,185

M3a165,166

P1a68,165,166

TDM207

A3g186

P1b173

A2185

M3f212,213

P1b173

A3h186

P1h173,185

A3c173

M6216

P1h173

A4a188

P1k218

A3e173

M3f

212,213

161

P1h185

185

218

comparison to other

PTM

191

116

S1b157

185

163,164

S2c

159

stability

P1k218

A5a A5b191

A2 A3a186

A4a A5b173,193

A3a186 A3c173,186

A5c191,192

A3c173,186

A5c173

A3d186

191,192

A5d

196

186

196

A3d A3e

162,195

CBR

207

TDM

212,213

A3e173,186

C1b

173,186

C1b

M3f

188

186

162

A3f186

CBR

207

A3f

A3g186

TDM

186

213

A3g

A3h186

M3f

186

216

A3h

A4a188

M6

188

A4a A5a189191

A5a190,191 A5b173,190,191,193

A5b173,189191,193

A5c173,190192

173,189192

A5d67,190192,194

67,189192,194,195

C1b196

196

CBR67,162

A5c

A5d

C1b

CBR

67,162

TDM207

207

M3a165,166

TDM

165,166

M3a M3f212,213 M6216 TPTP

162

MTDT 192

A5c

67,192

A5d

67,162

CBR

S1b157 S1f

S1b157

157,159

S1h

157

S1f

157

157

S1j

159

S1j

159

S4159 S5159 S6159

159

MTDT

165,166

P1a

192

A5c

S1b157 S1f

157

S1h

157

157

S1j

166

S1b157 157,159

S1f

157

S1h

157

S1j

S2c

P1a

S2c

S4159 S5159

S4159 S5159

M3a166

S4159 S5159

S6159 162

MTDT 165

P1a

165,166

P1a

165,166

M3a

162

MTDT162 A5c192 A5d67,192 CBR67,162

159

S2c

S6159 162

S1f

S1h

157

S1j

157,159 157

S1h

157

S2c

S1b157

157,159

S6159 MTDT162 P1a165,166 A5c192

CBR

67,192

A5d67,192

A5d

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Table 5. Continued mechanistic evidence TPTP (continued)

heterogeneous photodegradation photoproducts CBR

photocatalysts

stability

67,162

ANT

S1a

109

S1c

109

S2b

102

DCA

S1a S1c

109

107,156

161

S1i

161

S1j

161

S1c

104,105,107,109

S1c

107,156

S1f

107

S1f

63

S2a

S1a63,106 S1c S1f

105,107

107

S2a

63

S1h104

161

S1i S1j

S1f107

104,105,107,109

TDM207

S1b S1c

107,156

S1f

S1h104 S1i161

161

S1j161

S1j

S1k161 S1l161

S1m161

S1m161

S1m161

S1m161

TDM

161

S2a

161

63

S2b

S2a63

105

S2b105

173

P1b173

173

P1h173

173

A3c173

S2b

173

P1b

P1h

63

S2a

105

P1b

173

P1h

173

A3c

S1c107

156

161

S1i

161

S1a63,106,107

S1k S1l161

207

A3c

173

A3e A5b173

173

A3e A5b173

A3e173 A5b173

A5c173

A5c173

A5c173

TDM207

TDM207

S1c107

S1c107

S1c107

S1c107

S1c107

S1c107

S1f107

S1f107

S1f107

S1f107

S1f107

S1f107

H1a199

H1a199

H2a199

H2a199

H2b199 H3a199

H2b199 H3a199

H3b199

H3b199

H5a199

H5a199

S1a106

S1a63,106

S1a63,106

S1a63,106

S1a106

C1a

110

C1c

110

110

M1

PBIa

107

S1b

107,156

S1a63,106,107

S1k S1l161

S2b

AQ

P1j102 S1a63,106,107

S1k S1l161

105

DMA

S1c109 S2b109

P1j

156

104,105,107,109

S1h104

S1a106

PTE

S1b S1f

PTE

102

102

156

S1h104

S1a

106

102

S2b

102

S1f

PTE

109

S2b

S1c

102

S1c

109

P1j102 S1a63,106,107

S1c

S1a

109

PTE

104,105,107,109

106

S1a

P1j102 S1a106,107 S1b

P1a68

P1a 106

PTE

156

DCAC

M3a165,166 68

P1a

106

applicability

CBR

68

106

environmental

photocatalysts 67,162

M3a165,166 BP

comparison to other

A5a83

S1c

109

S2a

63

C1a

109

S1c S2a

S1a63,106

109

S1c109

63

S2a63

S1c

63

110

TDM207

S2a

110

C1a

C1c110 M1110

C1c110 M1110

A5a83

A5a83

A5a83

A5a83

72

A4b

PBIb

H672 P1a167

PBIc 156

S1b

104

156

S1b

156

S1b156

104

S1b

S1c S1f156

S1c S1f156

S1c S1f156

S1c104 S1f156

S1h104

S1h104

S1h104

S1h104

116

S3

116

S7 AYG

A5d67

A5d

RA NMQ

P1a167

67

74

A5d

104

116

116

S3

S3116

S3

116

116

S7

S7116

S7 162

MTDT

162

MTDT162

MTDT

1740

MTDT162


Chemical Reviews

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Table 5. Continued mechanistic evidence

heterogeneous photodegradation photoproducts 67,74

AYG (continued)

A5d

CBR RB

168

P1a

168

P1d

168

P1e

168

P1f

168

P1g

168

S1c

stability

162

167169

P1a

173

P1b

124

P1c

168

environmental

photocatalysts

applicability

CBR

A5d

162

CBR 109

S1c

167,169

P1a

173

P1b

124

P1c

80

169

P1a

124

P1c

143,175

P1e

124

P1m

124

P1e

P3a

P1e

141

P2a

CBR162

109

P1b173

167

P1e143

173

P1h173

124

P1n178

143,167

A3c173

167

S1c

P1a

P1b P1c

P1e

P1e P1f167,168

P1h P1m124

P1f P1g167

A3e173 A5b173

P1n178

P1g167,168

P1n178

P1h173

M8129

P1o

168

P1p

141

P1h

173

168

P1j

168

P2a

P1k

P2b141,181

P1m124

141

P2c

182

178

P1n

141

124

P2a

P1m

181

178

P2b

P1n

124,182

P3a

P2b141,181

173

P2c141

173

A3c

A3e A5b173

P3a124,182 P3b182

H1a144,198,200

P2a141

H1a144,198

A3c173

H1b

144,198

H1c

144,198

H1d

144,198

H1e

144,198

H1f

144,198

P2b

141

P2c

124,182

P3a

182

P3b

173

A3c

207

TDM

A3e173

217

A5b173

129

H1a144,200

M7 M8

129

H2b144,200

129

H3a144

M9a

M9b

H1g H1h144

173

A3e A5b173

H3b144 H4142

H2a144

A5c173

H5c144

H2b144,200

H1a144,198200

H7a121

H2c144

H1b144,198

H7b121

H3a144

H1c144,198

H7c121

H3b144

H1d144,198

H7d121

H4142

H1e144,198

H7e121

H5a144,201 H5b144,201

H1f144,198 H1g144,198

H7f121 TDM207

H5c144,201

H1h144

M2208

H7a121

H2a144,199

M7217

H7b121

H2b144,199,200

M10a219

121

H7c

121

144 144,199

H3a

H7e121

H3b144,199

H7f M7217

H4142 H5a144,199,201

M8129

H5b144,201

M9a

129

H5c144,201

129

H7a121

M9b

219

H7b121

219

H7c121

M10a

M10b

M10b219

H2c

H7d

121

M9b129

P2a

P3b182

P1o P1p168 141,181

M9a129

141

P3a P3b182 144,198

168

173

143

A5d74

P1j P1k168 168

80,143,167,168,175

comparison to other 67

162

109

P1d

photocatalysts

H7d121 H7e121 H7f121 TDM207 M2208 M7217 M8129 1741


Chemical Reviews

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Table 5. Continued mechanistic evidence RB (continued)

heterogeneous photodegradation photoproducts M9a

photocatalysts

stability

comparison to other

environmental

photocatalysts

applicability

129

M9b129 M10a219 M10b219 RF

123

P1a123,170

123

123

P1a

P1d

176

P1e

123

P1d

176

P1e

123

P2a141 P2c

P1j123

P1j123

AMT204

176

P1o

141

P2a

141

P2b

141

P2c

P1k

H4

203,204

ATZ ATT204

P1k

H2b

141

P1g P1i123

170,176

200

P2b

P1g P1i123

176

P2a141

141

142 203

ATZ TNT209

P2a141

ATT204

141

AMT204

200

TNT209 M5214,215

P2b P2c

H1a H2b200 H1a144

214,215

H2b144

M5

P1o

170,176

H3a144

P1p

170

H3b144

141

H4142

141

H5c144

P2a

P2b

187

A3g H1a144,200

141

P2c P3a170

H7a121 H7b121

H2b144,200

A3g187

H7c121

144

H3a

144

H3b

142

H4

144,202

H5c

121

H7a

121

197

H7d121

C2

144,200

H7e121

144,200

H7f121

144

M2208

144

M7217

H1a

H2b H3a

H3b

142

H7b H7c121

H4 H5c144,202

H7d121

H7a121

H7e121

H7b121

H7f121

H7c121

M7217

H7d121

M10a219

H7e121

M10b219

H7f121

ATZ203,204

141

M10a219 M10b219

ATZ203,204 ATT204 AMT204 M2208 TNT209 M3a170 M3b170 M3c170 M3d170 M3e170 M5214,215 M7217 M10a219 M10b219 MB

158

S1d158

S1d158

S1d158

S3116

P1n178

158

158

158

158

116

A5b193

S1d S1e

158

S1e

158

S1e

158

S1e

158

S7

167

S1f S3116

S1f S3116

S1f S3116

S1f P1c124

P1a P1c124

S7116

S7116

S7116

P1m124

P1m124

167

P1a

124

P1c

178

P1n

167

P1a

124

P1c

143

P1e

167

P1a

124

P3a

124

P1n178 P2b181

P1c

124

P3a124,182

P1m

1742


Chemical Reviews

REVIEW

Table 5. Continued mechanistic evidence MB (continued)

181

heterogeneous photodegradation photoproducts 124

photocatalysts

stability

178

comparison to other

environmental

photocatalysts

applicability

182

P2b

P1m

P1n

P3b

P3a124,182

P1n178

P2b181

A3a186

182

P3b

181

P2b

124,182

A3c186

182

A3d186

P3a

124,182

P3a

P3b

182

A3e186

P3b A1

184

A3f186

186

A3g186

186

A3c A3d186

A3h186 A5a191

A3e186

A5b191,193

A3a

186

A5c191

186

A5d67,191

186

M2208

A3f

A3g

A3h

191

A5a

A5b191,193 A5c191 A5d67,191 M2208 DDT211

MG CH1

130

P3c

DDT211 P3c130

P3c

143 205

ATZ 130

P3c

PP2b

143

P1e

205

PP1

P3c130 143

P1e

CH2 PP2a

130

ATZ

130

205

ATZ

ATZ

130

P3c

P1e143

P1e

205

P3c

P3c

NDMA90 DCANa90

NDMA90 DCANa90

P1g90 P3d90

NDMA90 DCANa90

P1g90

P1g90

P1g90

P3d90

P3d90

P3d90

A4b90

A4b90

A4b90

PP2c

P3c130

P3c130

P3c130

P3c130

PP2d

P3c130

P3c130

P3c130

P3c130

P1a167

P1a167

P1a172 P3c183

P1a172 P3c183

P1a172 P3c183

ATZ206

ATZ206

ATZ206

AMT206

AMT206

AMT206

AMT206

TNT210

TNT210

TNT210

PP2g

TNT210

TNT210

TNT210

PP2h

210

210

TNT210

PP2e PP2f

PP2i PP2j

TNT 130

P3c

88

P1g P1l88

ATZ206 206

AMT PP2k

150

P1g

ATZ206 AMT206

130

P3c

88

P1g P1l88

206

P1g P1l88

ATZ206

ATZ AMT206

ATZ206

206

AMT

174

174

P1b

P1g150

A5b174

A5b174

174

A5c174

88

P1g88 P1l88 AMT206

AMT

P1g150

150

P1g

PP2l

P1g P1l88

P1g P1l88

P1g88 P1l88

P1g88 P1l88

PP2m

P1g88

P1g88

P1g88

P1g88

88

P1l PP2n

88

88

P1l

P1l88

P1l 90

NDMA

90

DCANa

P3c183

ATZ206

206

P1b

P1a172 ATZ206

P3c130

P3c

88

A5c 88

P1a167 P1a172

TNT

130

ATZ205

130

90

NDMA

90

DCANa 1743

90

NDMA90

90

DCANa90

P1g

P3d


Chemical Reviews

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Table 5. Continued mechanistic evidence

heterogeneous photodegradation photoproducts 90

PP2n (continued)

PP3

stability

90

environmental

photocatalysts

applicability

90

P1g

P1g

P3d90

P1g

P3d90

90

90

A4b90

A4b

151

M4

M4

CN152

PC1a

160

S1g

PC1b

160

S1g

PC1c

CN152 160

S1g160

S1g160

S1g160

160

160

160

S1g160

S1g S1g

146

PC1e

Na2S147

Na2S147

PC1f

147

147

Na2S

Na2S

172

P1a

S1g

S1g

146

Na2S Na2S2O3146

PC1d

146

Na2S Na2S2O3146

Na2S Na2S2O3146 Na2S147

167,172

P1a

172

Na2S147

P1a

167,172

P1a167,172

167

P1e167

167

P1f167

P1a

P1e P1f

167

P1g167

P1g PC1g

comparison to other

P3d90 A4b 151

photocatalysts

153

153

S1h S1n153

S1h S1n153

P1a P1b148

S1h S1n153

S1h S1n153

S1h153 S1n153

P1a148,172

P1a148,171,172

P1g94,148

P1a171,172

P1a171

P1a148,172

148

148

P1b

P1b

148

94,148,171

P1g

P1g

153

148,172

179

P1n

179

P1p

171

A3b

171

P1g

171

P1j

171

P1j

P1k

153

P1k

171

171

P1n179

153

171

P1g

171

P1j

P1k

171

P1b148 P1g94,148 A3b153

171

P1o

P1o

A3b153

A3b153

171

P1o P1p179 A3b153 PC1h

Na2S147

Na2S147

Na2S147

Na2S2O395

Na2S2O395

S1o95 PC1i

ATZ205

PC1j

P1a167

PC1k

S1o95 ATZ205

ATZ205

ATZ205

ATZ205

P1a167

S1h153 S1n153

S1h153 S1n153

S1h153 S1n153

S1h153 S1n153

S1h153 S1n153

A3b153

P1e177

P1o98,180

P1o98,180

A3b153

P1g177

A3b153

A3b153

P1k177 P1o98,177,180 A3b153 PC1l

148

Na2S147,155

148

P1b P1g148

Na2S2O395,155 95

P1b P1g94,148

P1j154

P1a148,167

P1j154

P1a

S1o

P1a148

P1b148

148

154

P1j

P1a148,167 P1b148

94,148

P1g94,148

P1g

154

P1j154

P1j PC1m

Na2S

Na2S2O3 PC1n

Na2S 146

146,155

146

Na2S2O395,146,155 95 S1o Na2S146,155

Na2S

Na2S146,155

Na2S2O3146

Na2S2O395,146,155

146

Na2S

Na2S2O3146,155 S1o PC1o

Na2S2O395,155 S1o95

148

P1b

146

Na2S147,155

Na2S2O395,146,155

95

Na2S

155

S1o95 Na2S146,155 S1o95

94

Na2S155

P1g

Na2S2O3155

Na2S2O3155 1744


Chemical Reviews

REVIEW

Table 5. Continued mechanistic evidence PC1o (continued) PC1p P1j154

PC1q

heterogeneous photodegradation photoproducts

photocatalysts

comparison to other

environmental

photocatalysts

applicability

stability

94

P1g Na2S155 Na2S2O3155 P1g94 P1j154 A5d67

94

P1g94 P1j154

P1g Na2S155 Na2S2O3155 P1g94 P1j154 A5d67

P1g94 P1j154

Na2S2O395 S1o95

PC2 PC3a

P2a97

PC3b

97

Na2S155 Na2S2O3155

Na2S2O395 S1o95 P2a97

Na2S155 Na2S2O3155

P2a97 P2a

155

Na2S Na2S2O3155 P2a97

P2a97

P2a97 97

Na2S155 Na2S2O3155 P2a97

P2a

after 200 min of irradiation, showing that only minor changes in the composition of the dissolved organic fraction occur, as expected for an oxidative process. Another approach to improve the sustainability and applicability of photocatalytic processes is coupling with a biological treatment.250 For this purpose, assays to assess the biocompatibility (toxicity/biodegradability) of samples are needed; however, very scarce information is available on the use of bioassays in processes involving organic photocatalysts. A. branii alga cultures has been used in toxicity tests, measuring its growth in the presence of treated and untreated samples with respect to a control.173 Alternatively, respirometric assays of activated sludge have been employed to determine toxicity and short-term biodegradability.74,162 Inhibition of the respiration of activated sludge is taken as an indicator to monitor the detoxification of solutions of two pesticides, methidathion and carbaryl, when irradiated in the presence of TPP, TPTP, or AYG (see Figure 42).162 Nearly complete detoxification of the methidathion containing sample is achieved after 3 h in the TPTP treatment. Likewise, Vibrio fischeri bacteria are particularly useful for samples showing only moderate toxicity, such as ferulic acid, as this method is more sensitive than those based on activated sludge.251 Thus, toxicity decreases during solar treatment of ferulic acid catalyzed by AYG, from 77% down to 11% at the end of the process.74 There is also a concomitant increase in the biodegradability of the treated sample, as indicated by the final value (t30W = 420 min) of the short time biological oxygen demand (BODst), which is ca. 1 order of magnitude higher than the initial measurement.

P2a97

for environmental purposes, especially because they can be used in aqueous medium, are activated by sunlight and require the presence of atmospheric oxygen as the only oxidizing reagent. The most important advantage of this approach is probably its versatility, as it is conceivable to design an appropriate photocatalyst for each specific problem, based on mechanistic knowledge. The most important drawback from the practical point of view might be associated with the limited stability of organic photocatalysts under the usual experimental conditions. Nevertheless, this can be circumvented by adsorption onto solid supports, which provides robust materials that can be easily separated from the reaction mixtures and reused in successive catalytic cycles. Among the issues requiring further efforts during the next years, heterogenization will be doubtless one of the most critical; a satisfactory solution of this aspect will make it possible to achieve substantial advancements in the field. After optimization of the procedures, the ultimate challenge will be the use of natural sunlight for the remediation of real wastewaters.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: (+34)963879444.

BIOGRAPHIES

5.3. Tabular Survey of the Endpoints Addressed with Different Photocatalyst/Pollutant Combinations

A comprehensive review of the different issues that have been dealt with during the studies performed on the oxidation of pollutants and model compounds with a variety of organic photocatalysts is provided in Table 5.

6. SUMMARY AND OUTLOOK Organic photocatalysts show potential to be used for the photooxidation of pollutants in the decontamination of wastewaters. They exhibit a wide structural diversity and operate through a variety of photochemical mechanisms. On this basis, organic photocatalysts constitute, in principle, an interesting tool

M. Luisa Marin was born in Valencia, Spain, in 1968. She did her Ph.D. at the University of Valencia working on the synthesis 1745


Chemical Reviews of natural products. In 1996 she moved to Imperial College, where she spent two years as postdoctoral Pierre and Marie Curie fellow. When she came back to Spain she moved to the Technical University of Valencia (Spain) and started working in the group of Prof. M. A. Miranda in the field of photochemistry, photophysics, and photobiology, being promoted to Associate Professor in 2004. Her current research interests are (a) molecular mechanisms of photocatalysis, phototoxicity, photoallergy, and photocarcinogenicity; (b) excited states as tools in bioorganic chemistry; and (c) supramolecular photobehavior within biomolecular hosts.

Lucas Santos-Juanes was born in Alcoy in 1978. He followed his engineering studies at the Technical University of Valencia (Spain), where he obtained his Ph.D. degree in 2008, working under the supervision of M. Luisa Marin and Antonio Arques. The main focus of his thesis was the use of organic sensitizers as photocatalysts for water remediation, a work that was awarded with the Ph.D. prize of the Technical University of Valencia. After spending a short research stay at INIFTA (CONICET) in La Plata (Argentina), in 2009 he obtained a postdoctoral position at CIESOL, a joint center of the University of Almeria-CIEMAT, where he is currently engaged in photocatalysis research.

Antonio Arques was born in Alcoy, Spain, in 1972. He studied Chemistry at the University of Alicante and did his Ph.D. at the Technical University of Valencia working on water treatment. Since then, he has been working in the group of Advanced Oxidation Processes of the Technical University of Valencia. In 2008, he was promoted to his current position of Associate Professor in the Department of Textile Engineering. His research is focused on solar photocatalytic processes for the

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remediation of polluted effluents (textile industry, pesticides, emerging pollutants, etc.).

Ana M. Amat is Professor of Chemistry at the Technical University of Valencia (Spain) and Head of the Advanced Oxidation Processes Group. She studied Pharmacy at the University of Valencia, where she obtained her Ph.D. degree in 1988, working on the photochemistry of carbonyl compounds in strongly acidic media. Since 1989 she works at the Technical University of Valencia, where she has been promoted to her current position as Full Professor in 2010. Her research is currently focused on photocatalysis and solar photodegradation of organic pollutants, eventually coupled with biodegradation processes, with special emphasis on the elucidation of reaction mechanisms.

Miguel A. Miranda is Professor of Organic Chemistry at the Technical University of Valencia (Spain) and Head of the Institute of Chemical Technology. He studied Chemistry at the University of Valencia and obtained his Ph.D. degree at the Autonomous University of Madrid in 1978, working at the Spanish National Research Council (CSIC). He spent postdoctoral periods at the University of Saarland and the University of W€urzburg (Germany) and was Associate Professor at the University of Valencia before accepting his present position in 1990. His research interests are mainly focused on photochemistry and photobiology. Dr. Miranda has received the Honda-Fujishima Lectureship Award of the Japanese Photochemistry Association (2007), the Janssen-Cilag Award for Organic Chemistry of the Spanish Royal Society of Chemistry (2008), and the Theodor F€orster Memorial Lectureship Award of the German Chemical Society and the Bunsen Society of Physical Chemistry (2010). He is currently the President of the European Society for Photobiology. 1746


Chemical Reviews

ACKNOWLEDGMENT Financial support from the Spanish Government (CTQ200913699, CTQ2009-13459-C05-03/PPQ, RIRRAF RETICS), and the Generalitat Valenciana (Prometeo Program) is gratefully acknowledged. Dedicated to Prof. Avelino Corma on the occasion of his 60th birthday. REFERENCES (1) United Nations Educational Science and Cultural Organization, UNESCO; 2006. (2) Food and Agriculture Organization, FAO; 2007. (3) Shon, H. K.; Vigneswaran, S.; Snyder, S. A. Crit. Rev. Environ. Sci. Technol. 2006, 36, 327. (4) Ren, S. J. Environ. Int. 2004, 30, 1151. (5) Comninellis, C.; Kapalka, A.; Malato, S.; Parsons, S. A.; Poulios, L.; Mantzavinos, D. J. Chem. Technol. Biotechnol. 2008, 83, 769. (6) Gogate, P. R.; Pandit, A. B. Adv. Environ. Res. 2004, 8, 501. (7) Gogate, P. R.; Pandit, A. B. Adv. Environ. Res. 2004, 8, 553. (8) Gottschalk, C. L. J. A. S. A. Ozonation of water and wastewater; Wiley, VCH: Weinheim, Germany, 2000. (9) Masten, S. J.; Davies, S. H. R. Environ. Sci. Technol. 1994, 28, A180. (10) Imamura, S. Ind. Eng. Chem. Res. 1999, 38, 1743. (11) Beltran, F. J.; Gonzalez, M.; Gonzalez, J. F. Water Res. 1997, 31, 2405. (12) Chen, G. H. Sep. Purif. Technol. 2004, 38, 11. (13) Martinez-Huitle, C. A.; Brillas, E. Appl. Catal., B 2009, 87, 105. (14) Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671. (15) Openl€ander, T. Photochemical purification of water and air. Advanced oxidation processes (AOPs): Principles, Reaction Mechanisms, Reactor Concepts; Wiley, VCH: Weinheim, Germany, 2003. (16) Cooper, W. J.; Cramer, C. J.; Martin, N. H.; Mezyk, S. P.; O’Shea, K. E.; von Sonntag, C. Chem. Rev. 2009, 109, 1302. (17) Pera-Titus, M.; Garcia-Molina, V.; Banos, M. A.; Gimenez, J.; Esplugas, S. Appl. Catal., B 2004, 47, 219. (18) Rajeshwar, K.; Osugi, M. E.; Chanmanee, W.; Chenthamarakshan, C. R.; Zanoni, M. V. B.; Kajitvichyanukul, P.; Krishnan-Ayer, R. J. Photochem. Photobiol., C 2008, 9, 171. (19) Chiron, S.; Fernandez-Alba, A.; Rodriguez, A.; Garcia-Calvo, E. Water Res. 2000, 34, 366. (20) Gonzalez, M. G.; Oliveros, E.; Worner, M.; Braun, A. M. J. Photochem. Photobiol., C 2004, 5, 225. (21) Glaze, W. H.; Kang, J. W.; Chapin, D. H. Ozone Sci. Eng. 1987, 9, 335. (22) Braslavsky, S. E. Pure Appl. Chem. 2007, 79, 293. (23) Fagnoni, M.; Dondi, D.; Ravelli, D.; Albini, A. Chem. Rev. 2007, 107, 2725. (24) Palmisano, G.; Augugliaro, V.; Pagliaro, M.; Palmisano, L. Chem. Commun. 2007, 3425. (25) Ravelli, D.; Dondi, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2009, 38, 1999. (26) Malato, S.; Fernandez-Iba~nez, P.; Maldonado, M. I.; Blanco, J.; Gernjak, W. Catal. Today 2009, 147, 1. (27) Blanco-Galvez, J.; Fernandez-Iba~nez, P.; Malato-Rodriguez, S. J. Sol. Energy Eng. 2007, 129, 4. (28) Malato, S.; Blanco, J.; Alarcon, D. C.; Maldonado, M. I.; Fernandez-Iba~nez, P.; Gernjak, W. Catal. Today 2007, 122, 137. (29) Malato, S.; Blanco, J.; Vidal, A.; Alarcon, D.; Maldonado, M. I.; Caceres, J.; Gernjak, W. Sol. Energy 2003, 75, 329. (30) Malato, S.; Blanco, J.; Vidal, A.; Richter, C. Appl. Catal., B 2002, 37, 1. (31) Gaya, U. I.; Abdullah, A. H. J. Photochem. Photobiol., C 2008, 9, 1. (32) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (33) Zhao, J. C.; Chen, C. C.; Ma, W. H. Top. Catal. 2005, 35, 269. (34) Fujishima, A.; Honda, K. Nature 1972, 238, 37.

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